I know it sounds stupid, but if the gasket in the multiport handle is messed up, enough to have water leaking around it, it can cause sand to blow into the pool when you first turn on the pump. I can't explain why, but I know that it happens in my pool about once per season, and that's when I know it's time to replace the gasket. It stops after the gasket is replaced.

You can get pool sand at Lowe's, Home Depot, wherever--just make sure it's pool sand, because playground sand is not the same stuff.

Janet