There is some code in WiredTiger that automatically rolls back the oldest transaction that's done any updates, if the cache gets stuck full:

https://github.com/wiredtiger/wiredtiger/blob/develop/src/evict/evict_lru.c#L2306

It is an unusual path, and logging something to the message event handler would make diagnosing such a failure mode easier. If logging a message in that case makes internal testing noisier we could consider adding a new verbose=rollback_messages or similar configuration string.