I'm probably not the best person to ask (there are people who have been on here a lot longer than me and who have a lot more encounters with "trolls"), but I'll give it a shot.
Basically it seems like there are certain people who adopt a particular persona online and write comments in order to cause/stir a reaction from others. In other words, they write comments that are not necessarily truly their own views in order to cause a particular reaction. One example might be a person writing inflammatory comments about a controversial subject, with the goal of purposely triggering an angry reaction from others. But that's just one example.
I don't think it's unique to imgflip. It seems to be routine to see that kind of behaviour in most (if not all) places that have discussion groups, especially if there's some level of anonymity.
I don't know if that helps or not.