Presuming you don't have any current or past injuries, I'd double check that you're gripping the bar evenly and that your feet aren't pointed all wacky.
After that, try stretching your hips before you start deadlifting or during your general warm-up.
Magnificent Mobility is a great dvd for warm-up ideas. My favorite/one I find the most benefit from is the twisting reverse lunge (pictured above, but keep the torso more upright and focusing on rotating from the waist not reaching diagonally as shown.) Try 5-6 reps per side at the end of your general warm-up or before you start your deadlift warm-up sets.
Do you notice any similar shifting on any other exercises?