I'm afraid you're probably better off centering on the effects of Gregor's transformation than you are centering on why he is transformed. Why? Because no explanation is given. There is no evidence one way or another as to why Gregor is transformed. Any answer you come up with is only speculation.
Some would say that that's the point--that the world that's presented is absurd, and no reasons exist. That's one way of interpreting the story, though it's certainly not the only way.
However you interpret the story, though, there is nothing present in it that explains why Gregor is transformed.