It's very common when trying to stand up with a heavy load. Try doing some Lateral X-Band walks to strengthen the outer part of the glutes. Also this may be from a weakness in the knees. You can do some terminal knee extensions for before or after your lifting at least 3 days a week.
Other than that you're just going to have to force your knees out as best you can.
Just practice the movement without weight and focus on proper positioning. Take a video of yourself if you can so that you can see any issues that you're not aware of. Personally I don't move on to heavier weights unless my positioning is correct, so while that may slow down my overall progress I'd rather move efficiently. So if you're having this issue maybe try back tracking a bit and only working up to weights were your knees are in proper positions. It may be a little bit frustrating in the beginning to back track but I think it's worth it. Kelly Starrett talks about squatting mechanics a lot so check out some of his stuff if you haven't already. If I remember correctly, this article by Coach Everett also talks about this a bit: http://www.catalystathletics.com/art...?articleID=135