maintenance plan executing by agent I have a built-in, regular maintenance plan scheduled to run weekly.

The databases are SharePoint databases and a snap manager (netapp snap manager) backs up these databases. 

The rebuild maintenance plan is executed whenever the backup runs by the SQL server agent. 

Why and how is this job being called?
Is there a SQL event being throw when the snap manager starts the backup?  If so there could be an alert setup on that event which is setup to start the job.  Check the SQL Agent alerts to see if it is configured to capture anything.