As I understand it, they're big hairy brutes who live under bridges or in caves and attack unwary travellers. They tend to carry clubs and go around hitting people for no real reason other than to cause pain. They're pretty stupid, too.
Now, take that into the cyber-world, and you get a term for people who just "bash" things. There are trolls who "bash" macs, and trolls who bash windows. There are trolls who bash TV shows or sports.
Generally, it applies to those of us who delight in taking pot-shots at a given target. It fits us pretty well in this case.