Probably just nerves, getting excited, and forgetting to breath. There's two things that come to mind when you talk of going straight back. First, your feet are still sloppy and too far apart and thus that first, long step ends up being straight back. Second, its a lack of repetitons which only time will cure.
Next time you shadow box, visualize nothing but one of these bothersome opponents and box him 3 fast round in the mirror. Don't worry so much about punching, but as to how you move your feet. You can get in 15 rounds a week training your mind to react that way.