I'm afraid I can't shed any light.
When something like this happens I presume it's because of stuff I didn't see. There are subs and threads I just don't open for a variety of reasons so the likelihood of me witnessing the offender doing the offending is actually rather small.
I have only seen one explanation but couldn't tell if it was in jest:
He posted in the "If You Post In This Thread You Get Banned" thread?
Is that really a thing?