From my experience, there are a lot of clans and groups of friends who play FFA. If you kill one those members, they will tell everyone else to constantly kill you over and over until you quit. I find it pretty disgusting. Really not much you can do about it unfortunately unless you want to try to get in good favor with one of the larger groups.
Of course it could be something else entirely, but I'm willing to bet it is just because you made someone mad by attacking them.
I think that goes against what FFA should be, but there isn't much they can do about it unless they want to lose a majority of their FFA playerbase. Though you could make the argument that they are also losing players due to them not preventing this sort of behavior. But that is a topic for another thread.