Comment by Onawa

It depends on the game, but for those with some kind of marketplace or transferable currency, I'm guessing market manipulation is one possible reason.

For other games, maybe trying to interrupt some time limited event or tournament. Going all the way down the rabbit hole, if you're not already familiar take a look at how crazy things get in a game like EVE: Online.

Then of course there are the bored trolls and/or people who feel wronged by the game's developers or other players.