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.