Something is causing your log to bloat. One usual suspect is an index maintenance job. Stop shrinking your tran log every day first of all. As you can see it will simply grow again. The next course of action is to figure out what is causing the log to bloat. Look for index maintenance jobs, large batch processing jobs, rogue apps not closing their transactions, anything that would create a lot of modifications to data or indexes. That can help in troubleshooting. Is your database a publisher in a transactional replication scheme? Also try to narrow down the time frame.