Compare your form to that guy. There are so many things that could be causing what you're doing!
Before you do a leg workout, make sure you do some sort of cardio warmup and dynamic stretching (lunges are an example) and DON'T stretch non-warmed up muscles.
exrx is a great site! Check out all of the stuff they have and make sure your form is good on all of your exercises. If you're an anatomy/biology geek like I am you'll get lost on that site.