Following vitalistic theories of the day, many doctors believed some substance or energy that was vital energy itself was ejected or dissipated by ejaculation, so excessive ejaculation was potentially deadly from nerve waste alone.

Here's John Harvey Kellogg's explanation from his Plain Facts for Old and Young (1881):

RESULTS OF SECRET VICE

The physician rarely meets more forlorn objects than the victims of prolonged self-abuse. These unfortunate beings he meets every day of his life, and listens so often to the same story of shameful abuse and retributive suffering that he dreads to hear it repeated. In these cases, there is usually a horrid sameness—the same cause, the same inevitable results. In most cases, the patient need not utter a word, for the physician can read in his countenance his whole history, as can most other people at all conversant with the subject.

In order to secure the greatest completeness consistent with necessary brevity, we will describe the effects observed in males and those in females under separate heads, noticing the symptoms of each morbid condition in connection with its description.

Eminent Testimony.—An eminent English physician, Dr. Milton, who has treated many thousands of cases of this disease, remarks in a work upon the subject as follows:—

Anything beyond one emission a month requires attention. I know this statement has been impugned, but I am quite prepared to abide by it. I did not put it forward till I considered I had quite sufficient evidence in my hands to justify me in doing so.

An opinion prevails, as most of my readers are aware, among medical men, that a few emissions in youth do good instead of harm. It is difficult to understand how an unnatural evacuation can do good, except in the case of unnatural congestion. I have, however, convinced myself that the principle is wrong. Lads never really feel better for emissions; they very often feel decidedly worse. Occasionally they may fancy there is a sense of relief, but it is very much the same sort of relief that a drunkard feels from a dram. In early life the stomach may be repeatedly overloaded with impunity, but I suppose few would contend that overloading was therefore good. The fact is that emissions are invariably more or less injurious; not always visibly so in youth, nor susceptible of being assessed as to the damage inflicted by any given number of them, but still contributing, each in its turn, a mite toward the exhaustion and debility which the patient will one day complain of.

General Effects.—The many serious effects which follow the habit of self-abuse, in addition to those terrible local maladies already described, are the direct results of two causes in the male; viz., 1. Nervous exhaustion; 2. Loss of the seminal fluid.

There has been much discussion as to which one of these was the cause of the effects observed in these cases. Some have attributed all the evil to one cause, and some to the other. That the loss of semen is not the only cause, nor, perhaps, the chief source of injury, is proved by the fact that most deplorable effects of the vice are seen in children before puberty, and also in females, in whom no seminal discharge nor anything analogous to it occurs. In these cases, it is the nervous shock alone which works the evil.

Again, that the seminal fluid is the most highly vitalized of all the fluids of the body, and that its rapid production is at the expense of a most exhaustive effort on the part of the vital forces, is well attested by all physiologists. It is further believed by some eminent physicians that the seminal fluid is of great use in the body for building up and replenishing certain tissues, especially those of the nerves and brain, being absorbed after secretion. Though this view is not coincided in by all physiologists, it seems to be supported by the following facts:—

1. The composition of the nerves and that of spermatozoa is nearly identical.
2. Men from whom the testes have been removed before puberty, as in the case of eunuchs, are never fully developed as they would otherwise have been.

The nervous shock accompanying the exercise of the sexual organs—either natural or unnatural—is the most profound to which the system is subject. The whole nervous system is called into activity; and the effects are occasionally so strongly felt upon a weakened organism that death results in the very act. The subsequent exhaustion is necessarily proportionate to the excitement.

It need not be surprising, then, that the effects of the frequent operation of two such powerful influences combined should be so terrible as they are found to be.

General Debility.—Nervous exhaustion and the loss of the vivifying influence of the seminal fluid produce extreme mental and physical debility, which increases as the habit is practiced, and is continued by involuntary emissions after the habit ceases. If the patient's habits are sedentary, and if he had a delicate constitution at the start, his progress toward the grave will be fearfully rapid, especially if the habit were acquired young, as it most frequently is by such boys, they being generally precocious. Extreme emaciation, sallow or blotched skin, sunken eyes, surrounded by a dark or blue color, general weakness, dullness, weak back, stupidity, laziness, or indisposition to activity of any kind, wandering and illy defined pains, obscure and often terrible sensations, pain in back and limbs, sleeplessness, and a train of morbid symptoms too long to mention in detail, attend these sufferers.

Consumption.—It is well recognized by the medical profession that this vice is one of the most frequent causes of consumption. At least such would seem to be the declaration of experience, and the following statistical fact adds weight to the conclusion:—

Heart Disease.—Functional disease of the heart, indicated by excessive palpitation on the slightest exertion, is a very frequent symptom. Though it unfits the individual for labor, and causes him much suffering, he would be fortunate if he escaped with no disease of a more dangerous character.

Throat Affections.—There is no doubt that many of the affections of the throat in young men and older ones which pass under the name of "clergyman's sore throat" are the direct results of masturbation and emissions. [Only men who masturbate clear their throats.]

Epilepsy.—This disease has been traced to the vile habit under consideration in so many cases that it is now very certain that in many instances this is its origin. It is of frequent occurrence in those who have indulged in solitary vice or any other form of sexual excess. We have seen several cases of this kind.

Failure of Special Senses.—Dimness of vision, amaurosis, spots before the eyes, with other forms of ocular weakness, are common results of this vice. The same degeneration and premature failure occur in the organs of hearing. In fact, sensibility of all the senses becomes in some measure diminished in old cases.

Spinal Irritation.—Irritation of the spinal cord, with its resultant evils, is one of the most common of the nervous affections originating in this cause. Tenderness of the spine, numerous pains in the limbs, and spasmodic twitching of the muscles, are some of its results. Paralysis, partial or complete, of the lower limbs, and even of the whole body, is not a rare occurrence. We have seen two cases in which this was well marked. Both patients were small boys and began to excite the genital organs at a very early age. In one, the paralytic condition was complete when he was held erect. The head fell forward, the arms and limbs hung down helpless, the eyes rolled upward, and the saliva dribbled from his mouth. When lying flat upon his back, he had considerable control of his limbs. In this case, a condition of priapism seems to have existed almost from birth, owing to congenital phimosis. His condition was somewhat improved by circumcision. In the other case, in which phimosis also existed, there was paralysis of a few of the muscles of the leg, which produced club-foot. Circumcision was also performed in this case and the child returned in a few weeks completely cured, without any other application, though it had previously been treated in a great variety of ways without success, all the usual remedies for club-foot proving ineffectual. Both of these cases appeared in the clinic of Dr. Sayre at Bellevue Hospital, and were operated upon by him.

Insanity.—That solitary vice is one of the most common causes of insanity, is a fact too well established to need demonstration here. Every lunatic asylum furnishes numerous illustrations of the fact. "Authors are universally agreed, from Galen down to the present day, about the pernicious influence of this enervating indulgence, and its strong propensity to generate the very worst and most formidable kinds of insanity. It has frequently been known to occasion speedy, and even instant, insanity." ^((51. Arnold.)^)

"Religious insanity," so-called, may justly be attributed to this cause in a great proportion of cases. The individual is conscience-smitten in view of his horrid sins, and a view of his terrible condition—ruined for both worlds, he fears—goads him to despair, and his weakened intellect fails; reason is dethroned, and he becomes a hopeless lunatic. His friends, knowing nothing of the real cause of his mysterious confessions of terrible sin, think him over-conscientious, and lay the blame of his insanity upon religion, when it is solely the result of his vicious habits, of which they are ignorant.

His last point is interestingly psychological, but it's not the line of reason pursued by others. There are a lot of references in /r/Intactivists/wiki: annotated timeline of genital surgery.

The more compelling idea appears to have been that phimosis caused insanity. This idea was bolstered by Charles Guiteau's 1882 autopsy finding that he had had phimosis. He was the Illinois preacher, writer, lawyer and insane psychopath who assassinated president Garfield. This must have been an extremely influential case report especially in this time before evidence based medicine. This information was much more widely disseminated than medical surgery promotion for being news rather than the "research" produced by a growing number of anti-foreskin/pro-surgery activists writing in the medical literature: Hutchinson, Sayre, Remondino, Kellogg, Pratt, etc.

The idea that phimosis could cause insanity fed into wildly exaggerated fears about phimosis and the eventual confusion even by doctors (who should obviously have known better) between common, normal developmental (non-fibrotic) phimosis and pathological/fibrotic phimosis, a rare disorder. Developmental phimosis or physiological phimosis is the normal condition of the penis at birth and in early development (until the first to third year of life for most boys).

Here is a medical journal article explaining that medical misunderstanding (confusing developmental and pathological phimosis) from the perspective of the New Zealand medical community: The universal condition: medical constructions of 'congenital phimosis' in twentieth century New Zealand and their implications for child rearing. Watson. Health History. 2014. (Link requires access to EBSCO to read more than the abstract unfortunately.)

New Zealand's experience with medicalized foreskin excision is very similar to that of the UK. As that paper relates, the British study, Gairdner (1949) pointed out that on average 16 children per year had been dying of the surgery in the UK (and had been for many years at that point). That information was so shocking that it got everyone's attention, but likely the most important effect of the paper was informing the medical community that infantile phimosis was developmental rather than pathological phimosis, the common misunderstanding of the time (and one that persists still among many foreskin-ignorant doctors in the US).

Interestingly while the UK and New Zealand quit performing non-therapeutic, preventative foreskin destruction around 1949 as a reaction to Gairdner's research, Australia kept up the tradition a lot longer. The research that brought about the end of the promotion of medical foreskin excision in Australia was Leitch (1970).