Bullies -- people just looking to tear apart questions -- always have lower cost to answer and higher reward for answering than people looking to be helpful.

That said, the SO moderation was so awful I don't think it's correct to blame the downfall on the bully dynamic even if it was clearly present and might have eventually overrun the platform. I used to joke that an answer wasn't uniquely useful unless it had been locked as duplicate, but it wasn't really a joke: I kept a tally on a sticky note and of the posts I found useful, incorrect duplicate flags outnumbered open questions.