These two altcoins have historically followed each other very closely in terms of price until recently. As Bitcoin rises and altcoin prices have fallen, we have seen Peercoin ( PPC ) drop below Namecoin ( NMC ) significantly.
I believe the explanation for this is quite simple: there are more Peercoins in the market than Namecoins, therefore there are more potential sellers.
According to https://coinmarketcap.com/ the current PPC supply sits at around 21 million. The same site states the current available amount of Namecoins is around 9 million.