Dear MR or Modam:
I think you dont have the CHECK VALVE in the discharg of pumps. yor prolem hapend in the time tath you dont close the shoted pump discharg valve .
the water circulate inthe shoted pump .and it cuse the rotation of shoted pump in on usual rotation side and probably cuse to open the impeller nut of shoted pump.
close the shoted pumps valves to solve the problem.