Unexplainable DBCC CHECKDB Command Output
The error message you're seeing:


  Sqlcmd: Warning: The last operation was terminated because the user pressed CTRL+C.


Occurs when sqlcmd is asked to quit while it is waiting for a response from SQL Server.  Most likely someone (or something) killed the spawned cmd.exe process that was running SQLCMD.exe, which resulting in that error message being directed to the SQL Server errorlog.

Instead of using SQLCMD to execute DBCC CHECKDB, you should simply setup a standard T-SQL job step to run the DBCC CHECKDB command directly.

Alternately, run Ola Hallengren's excellent free and open-source Maintenance Solution, or one of the other myriad solutions available.