In the earlier conversations I had with Chem_Geek regarding this situation, the possibility that you were breaking down stuff trapped on the filter came up. The fact that you had so much junk on the filter makes that possibility even more significant.

So . . . besides doing the 'chemistry experiment' above, you might want to
#1 - Change your sand, &
#2 - Use skimmer socks (Amazon link: Jed Industries 80-852 Pool Skimmer Sock )

BTW, if Chem_Geek's experiment works, you can get washing soda at Walmart, and can use Baquashok for the peroxide source. So the results may lead directly to a practical solution.