That'll happen when the transaction log is truncated without being backed up first. Several things can cause that I'm sure... you could have a space issue - SQL will proactively truncate the logs to prevent running out of space. You could have a process (scheduled job, external backup app etc...) issuing commands to do backups and/or truncate logs (i.e. if you have your recovery mode set to "SIMPLE" it'll truncate logs when backups are issued - you want FULL in most cases).
You can wander around in the logs in SQL Enterprise manager around that time and see if you see anything interesting looking around that time - under the "Management" folder under your SQL server in enterprise manager look under SQL Server Logs - you should be able to open that days looks and have a peek at what might have been happening around that time.
