In short, the unexpected Transaction Log File growth occurs majorly due to repeated transactions.
So now we know various reasons for the issues. Now how do we identify the cause? We can have several different approaches to this issue. Now let us isolate the issue one by one.
Step 1: Use the tool Exmon (How to use Exmon - http://www.msexchange.org/tutorials/Microsoft-Exchange-Server-User-Monitor.html ) to identify if any users mailbox is misbehaving. If any user who uses the CPU(%) more than 50% most of the time, then most likely you have identified the problematic user. Ask the user to quick any client/client activity, or if not possible way to reach them disable the account for some time. Observe your store and the transaction logs. (Hope you have wonder, the issue resolvedJ).
If your issue did not resolve, or you are not able to identify any unusual operations users from the above step, try doing the step2.
Step2: Use your exchange manager to verify the following parameters for each mailbox in problematic store (if you have not identified the database, they you may do this exercise in all the databse inside the storage group where you have issue).
Message Ops, Folder Ops, Total Ops
Sort with high/low and verify if any mailbox has huge difference from others. If you get one then that could be the most likely the problematic mailbox for this issue. Check for corrupted messages in outbox, or any unusual activities performed by this mailbox.
Step3: If you have more databases on a single storage group (Exchange 2003 and previous version), you may sometime have to dismount and observe the transaction log growth.
Step4: If you did not get any clue from above steps, you may now have to analyze your transaction logs. I would recommend you to contact Microsoft for further help or you may try the steps mentioned in Scott’s post - http://blogs.msdn.com/b/scottos/archive/2007/07/12/rough-and-tough-guide-to-identifying-patterns-in-ese-transaction-log-files.aspx
Hope you are atleast able to proceed further on your issue!