Most likely is just a way to mint untraceable coins to pay for that ticket to Bermuda at noon. But it could be as simple and innocent as clever pool hoping. Most pool have fixed this so growth as this point seems odd though.
Most of it was used previously to pool hop. Or atleast that's what I would have imagined as the market for clean coins was low. Atleast one miner had detailed their use of it for hoping in the gpumax thread if anyone is really curious, go spend some time reading through that thread.
The intended purpose of the GPUMax service is to provide clean coins at a price. Or atleast thats really its most useful feature. And its not something thats been a secret. So it makes no sense for everyone to act like they just stumbled on this super hidden use for it.
As far as 51% with it, not possible with the hash power there. The last time GPUmax exposed its hashing power a month or so back, it had roughly 1TH total capacity at the time.