I belive most of those PoW were unintentionally orphraned, but it could also be abused.

Sometimes your client wasn't able to get a freshly generated PoS into the blockchain. If you have other adresses that can PoS you are the only one that sees your longer chain and begins to PoS on top of your not acepted PoS block.

If you then resend your block(s) you orphran all other PoS-blocks if your chain has higher trust.
The more txt you have in your wallet it lags more and more and it becomes more likely that you block doesn't get transmitted on the first try and some people resend then manually.