Like you, I only saw this as background noise whilst involved with something else, was the one on the bottom trying to bite the other?
I read that as one trying to bite the other on the bottom for a minute …

I didn't see any biting, but I too wasn't exactly concentrating on it. It looked to me that Mcauley fell on Arnautovic then was a little slow to get off, this annoyed him and he reacted but nothing major, then Alaba took it all out of context and came flying in and escalated it from nothing to something.