I suspect the last straw was his now deleted post where he said it makes sense Hillary would defend Muslims since she's defended rapists.
There was that, plus some other recent posts in which he was being belligerent towards other posters. He was temp-banned way back in September, and there was talk of him being permabanned back then, but after he came back from the tempban, he really didn't post that frequently, so the issue faded. But eventually, even though he wasn't posting that much anymore, such a high percentage of the posts he was making were either racist or rude to other posters that it prompted Nym to swing the banhammer.