Jacuzzi tub drain overflow

When the jacuzzi tub in the MBR is used as a normal tub, it drains normally. But if we turn the jets on/off and then drain it, it pushes water through the overflow pipe in the unfinished basement below, getting the floor all wet. Why would this happen? One related factor is that we have a well and a water filter/backwash system that also drains into the same pipe near that very same junction in the basement. But that backwash operates in the early morning hours, never at the same time we are draining the tub.

leak

There is something wrong with the overflow connection. You probably have more water in the tub for the Jacuzzi, than when bathing normally.