A customer of us was affected by a large number of DRS vMotions and could find that using our vMotion/Storage vMotion report. That report tracks all vMotion and Storage vMotion activities and can be a great help to find large amounts of VM migrations and large vmdk data transfers. 
He noticed that all the vMotion tasks were triggered by the system (DRS) and at a certain time. Actually some minutes after restarting VMware vCenter.
Our support could help and we pointed the customer to the following VMware KB article 2107950
To check your vMotion/Storage vMotion activities try our report yourself: Sign Up to opvizor Free Healthcheck
