Because they realized they could actually exercise power over him for their own amusement.
Chris was and probably is extremely susceptible to being baited into doing all sorts of things to his own detriment, and would constantly end up being the punchline for elaborate pranks.
GFW Radio had a pretty good summary of the history, if you don't care to do deep research on the wiki. "This is probably something that in 20 years or 30 years, people will show this as a textbook case of a kind of incredibly despicable crime."
I'll be honest: I think a lot of the shit that's been pulled on him is inspired and actually pretty funny... until you sit back and realize that an actual person is the punchline for it all and that it's chronicled and videotaped for the whole world to see. The "he's actually a terrible person and deserves it" angle is a paper-thin excuse used by people to justify their power trips. It's not like they targeted Chris out of a sense of vigilante justice; it's simply because he'll give them the kind of reactions that they want.
I have to wonder why this thread was made.