Doors

[Hasbinbad]

So basically, Trolling is baiting a hook, and going at a slow to medium speed, dragging your line behind you and waiting for bites..

Which is what forum trolling is. Say something outlandish/evocative/controversial to a community (bait your hook and drag your line) and wait for bites (lulz & popcorn).

From this concept trolling forums originally is derived..
It was simply a homonym for a troll, and thus evolved the idea of a forum troll.