Same thing happened to me. My comment was “kid deserved it” on a video where, at school a kid called a teacher the n-word and then squared up to fight him. The teacher punched the kid in the face.
That was my comment and there were others saying pretty much the same thing both above and below my comment. 10,000 karma gone.
I’ve been banned from subs before but never from the whole site. Interestingly that was not my only account banned. My main account (which I never post on) was also banned and when I made a new account, that was banned as well.
My theory is that someone flagged me for something and then went to all my posts and also flagged them and a limit was reached that just autobans site wide. That’s the theory anyway.
The funny thing is that all my comments are pretty benign and I actively fight against racism and bigotry on Reddit. I was one of the good guys!