Gale Shaw is correct!

No offense to those who posted it, but the advice to schedule a shrink is bad advice. It may alleviate the symptoms. But it won't resolve the problem.

Most likely, there is a process that performs a huge select, insert, or update. If it happens daily, then you should easily be able to deduce what time and then narrow it down from there and turn on a trace when you have a window, as to when it occurs. then it will take a little detective work to determine the process that is causing the growth.

If a lot or or a large transaction is occurring during your backup, that would also cause the transaction log to grow as the DB must have somewhere to hold the records until the backup completes.

Ceddy