A common mistake people tend to make with their lunge, or single leg squat variations in general, is caving or falling over as they go down or come out of the bottom. This can be due to abdominal, low back or hip weakness, improper breathing or just poor neuromuscular coordination. Whatever the reason, it is important to stay tall during the lunge (looking at the reverse lunge here) while driving through the hip. Here is a great exercise to help teach proper hip drive while staying tall. It is also just a killer exercise in general that can be loaded real easily and also hits the entire core real nice. Peep the vid.
Rosencutter Ultra Fitness