> They don't know why they've been banned.

The absence of reason makes me wonder a lot of possibilities. Maybe google bans people who use ad-blockers, maybe they didn't explicitly set that way but some of their algorithms must have figured out these users should go. I know, it is extremely unlikely but we're free to come up with reasons if google doesn't give one. Don't sit there thinking it won't happen to you(I was like that up until recently), get a domain name and transfer all important accounts to it. And do a Google takeout.