all of the databases recovery models changed
Recovery model is changed by a specific ALTER DATABASE statement, and as far as I've seen, there's never been a bug that would change it.

Some common sources of that statement include:


Backup software - some 3rd party backup software will put the database in simple recovery model, especially virtualization backup tools that just do snapshot backups rather than log backups.
Data warehouse load scripts - I've seen ETL processes that would flip the database into simple recovery model to eliminate the need for transaction log backups. (Not saying that's a good idea, just saying that I've seen it.)
Log space reduction scripts - sometimes folks try to shrink their transaction logs, and run into difficulty, so they flip the database into simple and then flip it back.