There is a pool I service weekly, where there has been a problem with the system blowing back when it is shut off. Its a very unique pool. It appears to be a home made inground, where they built the walls out of cinder block and troweled a sand bottom. Then they put a vinyl liner in it. Instead of a skimmer in the wall, there is a small skimmer that is inside the pool and connects to an 1 1/2 port in the wall. There is one return. Hayward Super Pump with a Hayward Sand filter.
Every time the system is shut down, there is so much blowback the skimmer basket gets blown out into the pool. It seems to be air blowing back. When I turn it off and rotate the multiport to backwash, I get a blast of air out the waste line. So why is this pressure building up? I'm pretty sure the sand was changed in the filter within the last two years. The filter itself doesn't look more than a few years old. Pump was just replaced about a week ago. System seems to run fine until it is shut down.