Hi! No problem picking on me and I'll be happy to answer the best I can. Keep in mind I'm no specialist in Celtic archaeology, so my information is limited to some of the articles and books I've read. I'm afraid I won't have too much to add to the articles already posted, but I'll sum up what I know/have heard. This is a topic that has come up in modern Druid discussions and texts, and generally it just references the archaeological findings and hypotheses.
As far as I understand it, the ancient Druids practiced human sacrifice on rare occasions, and this is one of the leading theories about the bog bodies. Some hypothesize that the bog people were willing victims who volunteered to sacrifice themselves for the continuity of society and nature. The assertion by the Romans, however, that the Druids practiced widespread and forced human sacrifice, has no evidence and is likely war propaganda (i.e., make the natives look really bad so people hate them more). The hypothesis that the victims were willing is based, from what I understand, partly on the mythology and partly on the expressions on many bog bodies- which tends to be rather peaceful (look at Tollund man). There has also been speculation that victims were drugged first, or even killed and then mutilated (i.e., killed by drugging and then stabbed after death). There are speculations that victims were convinced into volunteering by the lure of a year or two of being treated as an elite person, so that they were made into what was essentially a king or queen symbolically in order to be a fitting sacrifice for the gods/goddesses.
Others think that they were criminals that were executed, also probably by the Druids. The Druids were the judges of the time and presided over criminal offenses. It is, however, unlikely that the criminals were offered as sacrifices. As far as I understand, sacrifices were supposed to be high-ranking individuals- you were supposed to give your best from society.
Others think they were people who suffered from ergot poisoning and so went berzerk and appeared possessed, and so were killed. If I remember correctly at least one or two of them had evidence of chemicals from ergot in their bodies. Ergot is a fungus that grows on wheat and causes symptoms that would resemble possession or bewitched-ness and has been hypothesized to have had a significant role in the Salem Witch trials- having caused the symptoms of the victims. Ergot poisoning can cause hallucinations, sensations of your skin crawling, contortions of the body, etc. This may explain why some bodies are staked or tied into the bog. If they appeared possessed, people may have feared that they would reanimate.
I would imagine there are also some folks in there who were just murdered and the bog was a handy place to dump the victims.
I can definitively say that the bog bodies are not exclusively tied to Druidic cultures. The oldest bog bodies are really, really old (8000 BCE according to one of your archaeology sites above) and were long before the Druids came onto the scene (unlikely before 500 BCE and definitive by 200 BCE). The use of bogs for human sacrifice, capital punishment, or other disposal of human bodies seems to be a very long-standing practice that started before the Druids, was probably taken up by the Druids and used in certain ways (similar to how it is likely the Druids used Stonehenge, but it was built before them), and continued long after the Druids had been assimilated into Christianity (by about 600 CE in most areas).
