It could be anything, @Jennifer311 , and I don't wonder about it anymore. It's between the guest and Airbnb, and, if anything, I think "bullet dodged".
I did get a little insight into one of those years ago, thanks to my neighbour bringing up the name of someone I recognized as a former local who had kept trying to book here, opening new account after new account but getting zapped by the system.
The short version is that Airbnb was indeed shielding the host from the guest's past behaviour.
In another one, the guest told me himself that he was too young but trying to book anyway.
Best not to call Airbnb about it. They can't give out that sort of information. Just enjoy the quiet protection you have been afforded 🙂