Sorry, it hit the ridge in the bottom of the webbing, not the ground. Seriously, would a pro catcher just roll the ball back to the pitcher without easily tagging the runner out if he wasn't ABSOLUTELY positive he caught the ball, something he has done a million times in his career. He has got enough balls in the dirt to easily know the difference.
It could be an instinctive routine move to try to sell a strike out call to an umpire on a half swing on a low pitch. Pierzynski didn't make a half swing, but the catcher may not have known that since he's concentrating on the ball, not the batter. In any event it's a bonehead move by a third string catcher who must have known that his glove was in the dirt when he gloved the ball -- making it too close for assumptions.