There must be a term for this phenomenon somewhere already; but here's my theory, by me:
First of all, there's at least some of it written into the original ideology. Sure, Original Texts get spun off into all sorts of wacky fanfics until you can hardly recognize the source material anymore; but still, there're usually germs there. And the more black/white thinking and calls for violence, even metaphorical violence, there are in the original, the faster and bloodier the deterioration will probably be (not always; much also depends on the most visible and charismatic proponents). Even the most benign, peaceful, All-Loving Text/ideology can get twisted to evil lunatics' purposes, of course; but, if there's a gun in the first act, it's gonna go off by the third.
Then what happens is, once the Movement gets past a certain threshold of members, the frootbats are likely to take over the orchard. The reason for this is threefold:
1) they drive all the saner people away either through bullying or sheer dint of their toxicity
2) they have the most need for attention, and thus get it, in spades, from the followers, from the opposition, and from the public audience.
3) they have deep unconscious impulses fuelling their attachment to the Movement, which makes them more zealous than the more balanced people who were in the Movement as a means to an end, not the end itself. Plus also eventually the saner people, who do not, for ethical reasons, have a constant vampiric tap on other people for to fortify their constant inexplicable stream of energy, eventually burn out.
This is of course in addition to the usual "telephone" game of relatively benign misunderstandings, the petty power dramas, the lack of organizational skills, the sheer "oops, we never thought of that."
but mostly, I feel, if the whole "thesis/antithesis/synthesis" dealio had allowed for the FROOTBAT factor, well...we'd still probably be pretty much where we are, dialectically speaking, but we'd be more ironically self-aware about it.
well, no. not that.
that needs more work.
never mind; talk amongst yourselves.