why is pog going down?

First of all, one assumption for the logic below: POG price discovery market is the paper gold market.

The main purpose of buying gold is to hedge US dollar currency/monetary risk.
Now it has turned out that POG has not and will not go up when the USD fundamentals are weakened by debt buildup and other factors.
So POG is not performing the function it used to and was assumed/thought to perform.
So it has become apparent that Paper gold is useless for the purpose it was bought for.
So paper gold holders have been selling paper gold.
And fewer and fewer new investors are buying paper gold.

So supply of paper gold overwhelms demand for paper gold.

So naturally POG, the price of paper gold, has been going down.

Manipulation might also play a role, along the way.