Motive 

I just heard Olly Steeds quote some line of bull about these people all having been Sacrificed To The Gods.  I'm not an expert, but I think that "ritual" is a catch-all for unimaginative archaeologists; I have no doubt that one researching modern America would say (coming uncommonly close to the truth) that the gas chamber and the electric chair were also elaborate religious rituals.  But it would be interesting to hear more about whether anything is known here.  My bet would be that if you wanted to kill someone, you'd drop him in the bog, and these people are just murder victims... I wonder if there's any way anyone will ever know more than that.