For me its a glute issue.. We used to call it the "butt rocket" (yeah I know, stupid, but whatever) You sit down and back, relax your glutes, then fire the butt rocket and blast off.
As far as proper box squatting technique goes, I thought I read in Louie's book that you are supposed to relax your glutes and hamstrings once you sit down, than tighten up and explode off. Not stay tight through out. Can someone clarify that for me? And I must have been doing them wrong in the past...because I was able to do 100 more lbs than in my raw squat lol..