I assume that hose is hanging down from the outlet? If so, the hose is placing an extra bit of suction on the internal piping since the end of the hose is at a lower elevation than the plain outlet. That extra suction is probably getting the wort to flow faster and draw grain through the screen.
By the way, this is the reason that old time mash tuns had gooseneck taps on the outlets. It helps reduce the head difference and avoids placing too much stress on the grain bed due to higher than appropriate flow rates. If you raise you tube outlet elevation, you will achieve the same result.