There is probably a job scheduled to run which is causing the auto grow. Check the SQL Agent and see what jobs are scheduled. You can also fire up SQL Profiler and see what’s running on the SQL Server and when the autogrow event occurs.
If the autogrow is happening at night it’s probable that the maintenance plan is causing the autogrow. Normal database maintenance requires that the database have enough free space to contain the largest index being worked on twice, so if there is not enough free space the server will grow the database to make enough room.