They were account sharing bans.
No, you don’t know if people in your guild were account sharing/logging on to each other’s accounts. You don’t even really know if someone in your guild was logging on to some dude’s account to get him through a mage tower.
It might not even be something they are cognizant of, as far as being “forbidden”, when I was in a raiding guild in Vanilla it was done all the time, “Hey, I can’t play tonight do you guys need my druid?” I remember them laughing at me when I said, “Hey, you can’t share accounts and it could lead to banning?”
That is probably a LOT of them; to Blizzard it looks like account sharing if you login at a friend’s/boyfriend’s/girlfriend’s house and that friend also plays Wow; they would have no way of knowing that the other party was physically there and logging in. It would depend on if they did anything to filter out false positives.