There is a small rib at the back of the sump bilge, so water in the back moves from side to side as the boat heels and ends-up going down the sides but not into the sump box.
Look for rainwater leaks (stern cleats, deck fittings, etc.), also could be freshwater pipes if you have them running through the tramsom, or likely it could be the deck shower head connection). If it's salty, it could be the stuffing box.