If you are running with the log in ROLLFORWARD mode, which I assume to be
true,  your log filled up because the DB backup did not happen or complete
in time.  My system locked with the DB backup within seconds of finishing,
which would have free 'd up the LOG.   I schedule a daily DB backup and have
the DB BACKUP TRIGGER set to start a DB backup if the log is getting to
full.   Make sure your DB BACKUP TRIGGER is set low enough so the DB will
have enough time to complete before the LOG fills up.  I guess I didn't
review my setting and had increased the DB many times.
Matt