Why has Mark Teixiera's batting avg dropped so much?

He used to be a .300 hitter with 30 HR's and 100 RBI's almost each season with Texas. He even did real well with the Angels and Braves. But ever since he joined the Yankees, his average has dipped below .250 now. Why is that? Is there something wrong with his swing? Or is it the fact that every time he goes up there, he wants to hit a HR? What are your guys theories?