I've seen the log fill in Normal if you have a client running that's not sending commits often enough. Clients with NICs set to Autonegotiate, then ending up at 100/Half Duplex (or 10/Half duplex, a couple of times) will "pin" the log due to not getting to a commit point. Add in expiration on an active TSM server, and your log can fill up fast. Also, I believe the log doesn't commit during a database backup operation, so if that takes an extended period of time (wait for mount point, then large database to send), you can get the log pretty full.
Nick Cassimatis Today is the tomorrow of yesterday.