August, your knees are caving badly enough that your weight is actually shifting to the insides of your feet. You're going to have to do a much, much better job at keeping them out. Also--while I can't be absolutely positive due to the tight shot--but your stance appears to (maybe) be too wide, and this may be keeping you from both hitting proper depth and keeping the knees out.
My recommendation would be to strip 15 or 20 pounds off the bar, ensure proper stance width, work on hitting depth and jam those knees the hell out at the bottom.
-Stacey