It's hard to know the reasons why anyone votes the way they do, but my guess is that in addition to the by-now-expected great saves, his fellow pros noticed that De Gea drastically cut down on his errors. Specifically, it was the kind of error he didn't make that helped him greatly this season.
Remember the last-second equaliser at White Hart Lane, the one that led to Gary Neville's damning indictment? It was a mis-punch than happened to fall to Aaron Lennon, who crossed for Clint Dempsey to score. This is a "second phase" mistake, one where the keeper parries or pushes the ball back into play and then concedes. These are not always punished - had Lennon not happened to have been in the spot where the ball fell, there's no goal. There were two other second-phase mistakes as well, against Newcastle and (debatably) Swansea, where he parried shots back into the six-yard-box for goals.
This is not to excuse a second-phase mistake - a goal is a goal - and Eric Steele will have worked very hard with De Gea after each of these incidents. But those are the worst things De Gea did all season, and as 'clangers' go, they're just not that awful. It wouldn't have escaped the pros who voted - certainly not the Spurs players - that in that same game, he made a pair of brilliant saves which kept his team in the game.
Mistakes - BAD mistakes - happen. De Gea deserves credit for going through a highly pressurized season without having done anything like this:
Nothing against three excellent goalkeepers in Simon Mignolet, Ali Al-Habsi, and Pepe Reina. These mistakes happen. Joe Hart, last season's PFA vote-winner, allowed three balls to go right through him this season (at Sunderland, Southampton, and home to West Ham). It didn't happen to David De Gea in 2012-13.
As for his famed aerial weakness, it's still not the strength of his game. But opponents who bombarded him with crosses to exploit it largely got nothing from him. Stoke at the Brittania, Villa at Villa Park, and most recently, West Ham at Upton Park tested the Spaniard to and well beyond the laws of the game. He passed those tests.
There is a human element at play when humans vote, and De Gea's standout performance against Real Madrid at the Bernabeu will have been noted by his fellow pros. While the award is supposed to reflect league play, that stage is one that was not afforded to the excellent Hugo Lloris or Petr Cech, back to his best, to say nothing of Mignolet and Ben Foster, who toil in relative obscurity. Nonetheless, for me the PFA got it right. De Gea deserves his spot.