Scientifically? When sex is too much fun. No, really.

Thus Albert the Great, the medieval theologian-philosopher who probably spent the most energy on the subject of twins:

In this way too the birth of twins occurs...for there are some women and animals that delight overmuch in intercourse, and in that delight the womb is moved, when the sperm is poured in on her sensory nerves, and in that delight the sperm is divided. (trans. Resnick)

Albert, a good post-Crusades Christian polemicist, notes that twins and triplets are more common in Egypt because, you know, those wanton Egyptian women.

European women--and in fact, European men--don't get off the hook, either. A really interesting shift occurs in the De secretis mulierum (On the Secrets of Women) textual tradition. This anonymous work was falsely attributed to Albert, so today we call the author "Pseudo-Albert," and manuscripts of it often circulated with set commentaries. The text itself has this to say about the causes of twins and other "monsters":

Some monstrosity is caused by an irregular form of coitus. For if a man lies in an unusual manner when he is having sex with a woman, he creates a monster in nature. (trans. Lemay)

The tradition usually called Commentary A thanks to an accessible published English translation (no really...medievalists like Latin, but we are also lazy), clarifies:

Coitus that is performed standing up is irregular in nature (although the unlearned do not care about this)

Commentary B stresses the importance of foreplay...with description...probably by a celibate male...right.

Anyway!

This moralization of multiple births was the clear scholastic preference: the point that some action of the parents went "against nature," which preferred single births (and, as De secretis mulierum goes on to say, prefers males; yes, being female, period, is a birth defect in Aristotelian/medieval medical tradition). This results in either a weakness in the "formative power" of the fetus, ergo underdeveloped limbs or other birth defects; or a "superabundance" of material, as in the division of sperm.

But you can see signs in the same texts that the academic writers were balancing numerous traditions--moral, medical, and potentially more popular. Because De secretis mulierum goes right on to talk about the influence of constellations (that is, astrology) over monstrous births. This assuredly comes out of Galen, an ancient medical authority alongside Aristotle, but there are a few suggestions that it gets closer to "popular" tradition than the moralizing view. First, there is the SM discussion of the matter, which is just AWESOME:

Just as monstrous bodies are created, so are monstrous spirits...The example of twins, one of whom had the power in his right side that no matter where he was carried, he opened all bars and closings [in houses]. And the other had the power in his left side of closing all the locks that had been opened. The cause of this does not come from the matter, but from a special constellation of the heaven. But it does not derive from the constellation alone, but also from a special disposition of the matter to receive the impression.

So first, super-powered twins, but also you can see the need to balance different understandings of the cause of multiple births. The connection of monstrous births, twins and otherwise, to the heavens is picked up in non scholastic sources, too. While theologians waver between whether unusual births are the ordinary course of things from God's perfect actions (Augustine has no doubt) or God-sent portents of disaster (Isidore of Seville ultimately lands here), here's Florentine banker and chronicle-writer Giovanni Villani in early 14C:

There was born in Tarraio di Valdarno di sopra a boy with two bodies; he was bought to Florence and lived more than twenty days. Then he died in the Florentine hospital of Santa Maria della Scala, first one body and then the other. And when it was proposed to bring him alive to then priors, as a wonder, they refused to allow him in the palace, fearing and suspecting such a monster, which according to the ancients signifies future harm wherever it is born. (trans. Daston and Park)

Villani is talking specifically about conjoined twins here, which is actually a common feature of the medieval discussion. Indeed, conjoined twins utterly captured the medieval academic and popular imagination. The hospital mentioned by Villani drew quite the fame for it (Villani says he was told the story by his father), and even added a decorative plaque/statue of sorts as a marker.

The medieval fascination with conjoined twins, specifically, in the theological tradition goes back to Augustine. He recognized that conjoined twins are "good to think with," as scholars love to say--that is, that their particular way of being special offers the chance to think through theological and practical problems. For Augustine in City of God, that problem is first and foremost whether monstrous individuals and monstrous races (dog-headed people being the most famous today) are human or beasts:

The same account which is given of monstrous births in individual cases can be given of monstrous races. For God, the Creator of all, knows where and when each thing ought to be, or to have been created.

[...] Some years ago, quite within my own memory, a man was born in the East, double in his upper, but single in his lower half— having two heads, two chests, four hands, but one body and two feet like an ordinary man; and he lived so long that many had an opportunity of seeing him. But who could enumerate all the human births that have differed widely from their ascertained parents?

Augustine set the academic tone: that conjoined twins were descendants of Adam, that is, human, thus also subject to original sin and in need of salvation.

Deriving from this, scholastic theologians use conjoined twins to "think with." They spill A LOT of ink on pastoral questions of one baptism versus two, the nature of marriage (whether conjoined twins can have sex without inherently committing adultery), the resurrection of the body (if the resurrected body is the perfect body, will they be separate?), and even the nature of the will and consent to sin.

For the theologians in Paris and Cologne, this discussion was pretty much entirely theoretical. Accounts of the births of conjoined twins like Villani's exist, but are rare, and survival was even rarer. (Also, it's posssible that the mother frequently died in such cases, because sometimes it is explicitly noted that she did survive. No wonder the birth of conjoined twins was associated with disaster.) Irvin Resnick suggests that on the ground, most pastors struggled like the ones in Villani's tale with how to cope. Rubert of Schlettstadt, a 13C Dominican, indicates that some parish priests might be scared enough of what conjoined twins portended for their village to order them killed, although this could be a Dominican-secular (e.g. not in a mendicant/monastic order...roll with it) polemic thing.

The other thing to draw out of Villani's anecdote is the fascination with conjoined twins. This is already clear in the high medieval chronicle accounts from writers like Guibert of Nogent, or even the medical writers: they are always very careful to specify the nature of the conjoinment (how the twins are joined, usually expressed as what is double and what is not). There is no standard iconography of conjoined twins, either. All sorts of joinings, of women twins and male twins, are pictured!

But moving towards the late Middle Ages into the early modern era, conjoined twins in their monstrosity were already treated as spectacles: that is, something to be displayed and gawked at. Antonio Benivieni says that at the end of the fifteenth century, there were two sets of conjoined twins on display in Florence, one of whom had been brought from Milan evidently because the money for the "show" was better (or at least newer) in Tuscany. And yes, he is very concerned with their form.

The major, crucial perspective that is lost in lack of sources is, of course, what did the peasant farmer or urban artisan family think of suddenly have two babies instead of one? The jury is in on the question of did medieval parents love their children (duh), but the medieval family was also a crucial economic unit. Were twins also an extra burden, an extra mouth to feed during the unproductive first years? Or a doubling of potential future workforce? /u/itsallfolklore might be able to poke through later-recorded folktales for some evidence here--I don't have the skillset to back-interpret folklore correctly, although I'd like to get there someday--so for now we just have to rest in the silence of the sources on that question.