When you say you're doing heavy snatches at the end of your session, do you mean that you're doing snatches towards the end of your entire session, or are they after your snatch block? In general, you want to execute the lifts that tax your nervous system the most -- the snatches and clean and jerks -- first because you're fresh. If you're doing the snatches at the end of your workout, then your fatigue may be playing a big factor. However, if the heavy lifts are coming at the end of your snatch portion, then I would suggest the weight is too difficult (not necessarily too heavy) because it's causing you to deviate from optimal form.
Your footwork may be better from the hang because you're forced to be quicker and more authoritative. The same should happen when you start to do tall snatches again.
I would refrain from getting to a weight where you start to have this fault happen. It's important that you beat the proper motor pattern in.
Javier A. Sanjuan
Olympus Barbell Club
Dear God, please help me lift heavy and be awesome. Thanks. Amen.