He has issued a statement explaining the reasons for his withdrawal, which you can read here:
Sasquan had previously announced that the Hugo ballot in now at the printers, so Schubert's name will still appear, but he has indicated that he will refuse the award, should he win it.
I understand the reasons for his withdrawal and applaud his integrity. It cannot be easy to walk away from a major award, perhaps one that you have dreamed of someday winning. And this takes courage as well; like the others who have dropped off the Puppy slate, he will undoubtedly come in for a certain amount of angry barking from the kennels.