checkdb running constatly

I am running SQL Server 2005 SP1 (9.0.2047).  There are about 55 databases on the server.  I noticed recently in the SQL Server Logs that CHECKDB was running almost constantly all day, against all of the databases.  I run DBCC CHECKDB in a batch job daily at 1:00 A.M., but can't explain why it is running throughout the day.  Needless to say, this is effecting database performance.  I have looked everywhere I know, and see nothing that leads me to why this is occuring.  What could be causing this to happen?

Thanks,

Terry