He was banned by theymos for plagiarism. Someone asked about this specific case (temp ban with 2 years signature ban) and theymos replied to it:
Everything is case-by-case.
IIRC, theymos/a mod already stated that plagiarism is plagiarism and it doesn't matter how much posts/reputation someone has. Also, he didn't get a permaban like most users, so that's something.