A large transaction log over the evening hours with no processing, I believe is related to having the check boxes enabled in the SQL maintenance plan for the consistency and optimization checks. In a nutshell these configurations execute DBCC CHECKDB and DBCC DBREINDEX respectfully. I believe you should check these configurations and disable these options and review the log size the following day if no scheduling changes are commit...
If the SQL maintenance plan does not have the consistency and optimization checks enabled, then I would research the situation further. It may be worth monitoring the server during the entire night on a first hand basis to ensure unexpected users are not using the system or use a tool like SQL Profiler that ships free with SQL Server and sequentially lists all of the transactions. Another option is to review a product by the name of Log Explorer from Lumigent which provides an intuitive interface to the transaction log and provides additional features not available natively to review the transaction log. I hope this answers your questions.
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