Sand During Start Up

At the end of last pool season, I noticed more and more sand on the bottom of the pool.

Yesterday, I took all the sand out of the filter and inspected the laterals; everything looked in order so I put new sand in and got everything ready for start up.

Today, I started the system up on backwash, then rinse, and then filter. I walked over to the pool and noticed about 1/2 cup of sand below the return line. So, I turned off the pump and let everything settle for a minute. I turned the pump back on and ran to the pool and saw sand shooting out of the return line, but only for a second. Why would sand be shooting out at first and then stop after a second or two?

I have 1.2 hp pump / Hayward HRV 24 filter

Google pointed to Hydropool.com for the exploded view of that filter and it looks like a side mount valve type. Don't know what to suggest as you said the problem gradually got worse last season. That kind of rules out the wrong sand, sand level too high, wrong plumbing, and the pump / filter sizing issues that always pop up on the forum. You took it apart and found nothing wrong. Have you opened the valve and inspected the spider gasket?
Thinking outside-the-box these filters do physically expand a little when the pump is running. If you think of the total force trying to blow the tank apart at 15 psi with the number of square inches of surface area, the tank will expand. I see my top mount valve move at least 1/8" between on and off. Never measured it. Is it possible this dimensional change is opening up a sneak path in the internal plumbing? I really doubt this.

Ideas anyone? I'll keep stewing about this.
Al