Update: my community trust ban was for "suspicious reviewing activity"
This morning me and several other people found they were banned. The common denominator seems to be that in the account page source there's a flag "banned_by_community_trust".
Turns out there's more info. I went to account > review your purchases, and I see a message stating that Amazon has "noticed some unusual reviewing activity", and that I can't contribute anything to Amazon.
The main takeaway seems to be that some of us are getting booted from Vine not because we violated Vine terms, but because we're not able to review which in turn means you can't be in Vine. So: it's probably not Vine booting us but some Amazon fraud detection algorithm that we get caught up in.
That also means that if you are blanket banned from Amazon reviewing, then Vine customer service cannot help with this. We need to contact community-help@amazon.com or jeff@amazon.com and briefly and politely explain that you do everything by the book and you think you got banned by mistake, and ask them to investigate whether the ban was in error.