Squat form check 70kg

[arjan874]

Last week I did 80kg squats, but felt a mild discomfort (not really pain, but it didn't feel right) in my lower back halfway in my first set. I did finish my workout however, but the discomfort could be felt the whole day. I watched multiple squat form video's and read some instructions from the book again. I've deloaded back to 70kg, widened my stance, reduced depth a bit and focused more on tightening my core. After these changed I felt REALLY sore after squatting, especially my glutes.
But I still shift forward in the bottom, could you tell me how I can prevent this? And do you have any other comments on my form (is my depth still sufficient)?

Here's a video of my friday workout with 70kg.

70kg - YouTube

[Tom Campitelli]

You are overextending your spine, which is probably what is pissing your back off, and leading with your chest. All of this is helping to throw the weight on to your toes. Narrow your stance to shoulder width. Point your chest at the ground. Drive your hips up. Stop arching your back so hard. You are terribly underweight. You need to eat a lot. This will not only help your lifting, it will help everything else, too. Those people who ask you with a concerned expression about your health? They will stop doing so when you are 40 or 50 pounds heavier.