When you dont shove your knees out hard enougth, it relax your hamstring= knees sliding forword.
There is nothing to explain because you are correct and tamvvtli's assertion that his knees are doing what they are supposed to be doing is not correct. This is the problem with the Internet and these technique boards of late. Lots of people providing "bad advices" to quote Gov. Schwarzenegger. While not nearly as bad as some videos, the OP's knees generally don't get set early enough and continue to drift slightly forward all the way down. Occasionally, they move forward at the bottom, too.
Tom,
What is your advice to correct this? I keep trying to get them to set and nothing seems to be working. I am starting to get frustrated. Please help!
Thank you
Firstly, your knees sliding forward problem is not terrible and you will be able to correct it quickly. You need to sit back and trust yourself at the bottom of the squat. The low bar back squat is a hip driven movement and trying to keep your chest up and your torso vertical will not work. You need to lean over a little more, sit back, and focus on driving the hips up. Don't be afraid to let your knees come forward early in the squat. They need to get over your toes before you go half way down and then they need to stay there until you are halfway up on the ascent. Look up Rip's article on active hip for some more specific advice and cues. I'd also like to see you in a pair of lifting shoes instead of Chucks.