This is still a controversial topic among archaeologists, so I will summarise the main competing theories and the evidence for them here. We now have several hundred of these "bog bodies" (most of them just skeletons), and several other than Tollund Man have been suggested as subjects for ritual sacrifice; part of the evidence is that the injuries deduced from an examination of TM's remains have parallels elsewhere in the record.

[1] Ritual sacrifice

There is excellent evidence that the deaths of some bog bodies were violent. Tollund Man was hanged, and the rope used to hang him was still around his neck when he was found; evidence for hanging rather than simple strangulation comes from a 2004 forensic examination of the back of his neck wound using an infrared camera. This found evidence of a V-shaped indentation characteristic of a rope digging into the flesh of the neck as a result of the weight of a dangling body.

There is a second example of a hanged bog body in Elling Woman, who was exhumed from the same bog. In another case, that of Borremose Man, death was certainly by strangulation but it's not clear whether the method was hanging or garrotting.

Other examples of violent death include:

• Cut throat: Graubelle Man, Kayhausen Boy

• Garrotted: Yde Girl

• Scalped and face crushed: Borremose Woman

• Decapitated: Osterby Man, Old Croghan Man

• Stabbed in the chest and then decapitated: Dätgen Man

• Hair and right arm chopped off before death: Huldremose Woman

With regard to the evidence for ritual murder rather than some form of judicial execution, the main evidence comes in a couple of forms.

One is the "hyper-violence" of some of the deaths. The most dramatic example is that of Lindow Man II, who had been bludgeoned over the head twice with an axe – one blow being fierce enough to drive bone fragments into his brain – but who had also been garrotted and had his throat cut. The archaeologist Miranda Aldhouse-Green, in her recent Bog Bodies Uncovered, stresses that this was "a theatrical killing." The strangulation and the throat cutting had happened simultaneously, yielding the effect of a "fountain of blood bursting from the throat under high pressure." She adds that "as a final act of violence, one of his killers pushed him hard in the small of the back with his knee, causing the kneeling man to topple face down in the water. It is just possible that he was kept hovering at the point of death for this last act, for he had ingested water... while he was still breathing." In the case of Kayhausen Boy, the victim had been hogtied in a highly elaborate manner, such that (says Aldhouse-Green), "any attempt to free his hands would have pulled on the bonds so that the band between his legs would tighten agonisingly on his genitals and simultaneously throttle him."

A second strand of evidence comes from examination of the contents of the bog bodies' stomachs. There is some evidence of specially-prepared last meals, most notably in the case of Lindow Man II, who had eaten a meal including mistletoe, the sacred plant of the druids. A second bog body, Graubelle Man, had traces of ergot in his stomach which would have caused him to suffer hallucinations, unbearable sensations of burning (St Anthony's Fire) and convulsions.

Thirdly, post-mortem mutilation of the bodies has been observed in some cases. A dramatic example is that of Old Croghan Man, found in Ireland – a remarkable physical specimen of 6ft5ins, who had been decapitated after death, his body chopped in half at the diaphragm, and his nipples sliced through. His upper arms had been pierced and twisted hazel withies had been drawn through the holes.

Fourthly, it has been suggested that the mere fact that these bodies were deposited in bogs which were known to have preservative properties suggests the killers wanted to prevent the souls of the dead from escaping their bodies to cause trouble on earth - that is, that their religious belief was that the soul could only depart a body that had been burned or decomposed. This is conjecture, but there is some intriguing evidence that some bog bodies were "pinioned" in place for unknown reasons. The bodies of "Queen Gunhild", Osterby Man and Dätgen Man had all been pinned down in their bogs with wooden hurdles.

Fifthly, there has been some speculation about the positioning of some bog bodies. Many of them had been stripped naked but for one or two possibly symbolic items – Tollund Man was nude but for a leather belt and a cap. One of the bodies pinioned with hurdles had been deliberately placed directly over a bog spring. In the case of Old Croghan Man, his body was found placed exactly along the boundary line dividing two major medieval Irish jurisdictions. It stretches my credulity somewhat to suppose such a border had existed ever since the middle Iron Age, when OCM was killed, but some archaeologists have found the coincidence worth remarking on.

Sixthly, there is good evidence that physical deformity of various sorts is over-represented in the bog bodies that have been recovered - suggesting that "special people" may have been deliberately chosen for sacrifice:

• Yde Girl had a marked curvature of the spine

• Kayhausen Boy had a malformed hip

• Lindow Man III had six fingers (or to be more precise, an extra thumb)

All this, anyway, suggests something far more than standard execution, but it is very important to remember that the bog bodies that show evidence of ritual are in a small minority overall; and a lot of the evidence that we depend on to make assessments of sacrifice comes from an early period (often pre 1950s) and was not written by trained archaeologists, which may make some of it suspect. Hence some archaeologists have urged caution. Van Der Sanden, in his "Bog bodies: underwater burials, sacrifices, and executions" (in Franceso Menotti and Aidan O’Sullivan [eds.] The Oxford Handbook of Wetland Archaeology. Oxford: Oxford University Press, 2013) comments that in general, Irish and Danish archaeologists are more willing to give credence to the ritual theory than are Dutch and German ones. So with all that borne in mind :

[2] Injuries caused by the recovery of the bodies from the peat

Most the the bog bodies recovered in the past few decades have been badly damaged by the peat extraction machinery used in modern harvesting. Large cutting blades are used to process the peat and these are capable of inflicting at least some of the terrible injuries noted above - the decapitations and the bodies sliced in half. However, these sorts of machine are not capable of causing the fine injuries noted above in the case of Old Croghan Man.

[3] Accidental death. The most extreme counter to the ritual murder hypothesis is the theory that the bog bodies are those of people who stumbled into a morass and got trapped. The ropes found around their necks, in this reading, were the product of clumsy attempts at rescue. I think it's hard to make this theory stand up in most of the more exotic cases outlined above.

tl;dr While it's not impossible archaeologists are attracted to dramatic, unusual explanations for the deaths of the bog bodies, the fine details of the discoveries, and the fact that similar types of death are reported from different locations is good support for the ritual sacrifice theory.

Note this earlier thread, though I don't think it answers the questions.