Because battle of the alpha male :v Or female cos equality :LL Or bituin cos tumblr. Who would back down from becoming alpha star? Fanpop be like the animal kingdom. tuktok user gonna get you all the betches. Alpha fanpopper gonna ipakita their manliness. Alpha fanpopper gonna ipakita dominance and make wallposts and polls. Bitches pag-ibig wallposts and polls.
Cuz people respond to negativity with madami negativity. Sad, but true. Even I'm guilty of this. It's not just Fanpop, though. You should see YouTube comments or IMDb message boards (before they decided to shut down the boards, of course). YouTube and IMDb have the worst people, though Fanpoppers are no saints, either.
It is not just Fanpop. You will see fights pretty much everywhere. On other websites too (which I have seen far worse) as well as in real life. Not everyone can get along with each other. If you want a specifc reason it varies. Some just do it for fun, others because they want to get out some of their tension as there are also those who get offended pretty easily or for no reason etc. Either immaturity or mental condition on a specific moment can play a crucial role in that !!!!