As long as the tablet is charging when the Tablet AC Adapter charger is connected, this is a normal operation. The slight movement is provided to allow the AC adapter connector plug to be easily detached from the tablet.
NOTE: If the tablet is not charging when the AC adapter charger is connected or if the AC adapter connector plug does not properly connect, the AC adapter unit may need to be replaced or service for the Tablet may be required. Begin the procedure to send your Sony product into a Sony facility to be repaired. Go to Parts and Accessories.