SQL Server 2000 Log file size

Am running several databases on SQL server 2000.
Have several overnight jobs for interface and maintenance, etc.
Log file on one specific database increases in size 10 fold overnight. Unable to trace why.
I have a job to truncate the logs before the final overnight backup. Several times a week the job hangs - on nights when log file expands, so it is a pain. Able to resolve the issue after the event, but do not understand why the log file increases in size (at that time) in the first place. No other activity on the server at the time.

Check if you have a long running transaction,

dbcc opentran('dbname')

If a transaction is open then backing up transaction log or truncating it does not really do much. For large data load consider using BULK_LOGGED recovery model. Or change data load transaction to multiple smaller transactions.

Do you rebuild index and/or update statistics in the db? Will use lot of log space if so.