Does anyone know what this pipe is in an HVAC system?
This looks like a vent pipe that would be hooked up to a ventilation fan in the bathroom.  If the bathroom door was closed, you might get quite a bit of air loss out of that pipe, but when the door is open, there wouldn't be as much pressure to push the A/C out of that room.

Those ventilation fans are not supposed to vent into the attic because their supposed purpose is to remove humid air, but incorrect installations are pretty common.