I have a group of 30+ friends who play minecraft, use a separate TeamSpeak server and they also regularly talk about getting banned without obvious reason.

And I was once banned from Dota2 for "intentional feeding" which I guess is just a fancy way of saying that the others in my team expected my skill level to be higher than it was.

These online ban systems really are random. And typically the "moderation" is fully automated. I predict soon GPT3 will write the message to inform you.