They want the price to drop so they can get BTC cheaper.
Mostly this.. otherwise why would they care?
Plus many of them don't understand BTC and have heard about Silk Road and Mt. Gox and didn't look much further.
Also a little bit of this:
http://www.psychologytoday.com/blog/your-online-secrets/201409/internet-trolls-are-narcissists-psychopaths-and-sadistsTrolls largely just enjoy trolling and BTC is a good place to do it, people might actually lose money off of their trolling, which they love!
Eventually people realize they are trolls, they get a bunch of negative trust votes.. and they switch to a new account that hasn't been blacklisted by the community yet.