Either more interest in general or current players getting better transactional offers against their previous MM's they were using. I used to use Direct Edge exclusively until Arca started giving better transnational fee's if I executed my orders through their ECN passively (providing them volume).
I dont have any solid reference for this - it was referenced in a stock I was a part of a couple years ago. From what I remember PUMA is not a retail market maker (not alined with any of the brokerages). PUMAs appearance may represent an institution or large player building a position.
Again all based on somebody else research a couple years ago. If I get some free time I'll see if I can dig up some references.