Unwanted Fresh Water Tank Fillup

Not every time we camp, but once in awhile.... Our fresh water holding tank will take on water even when we are not filling it the "normal way". I woke up this morning to find it 2/3 full - and that was empty 24 hours ago. We have no hose to fill it connected, it's apparently coming in somehow from the regular "City Water" connection. I have also noticed that when this has happened other times, the water pump for the tank will not run - the light comes on but the motor makes no noise (it just gets a quick press on, then off - but no noise).

Has anyone else had this issue? Any idea why this would happen and why it only happens about 30% of the time? Also is there a way to avoid this?

Thanks for any help, ya'll.