Squat check-hips shifting

[TS310]

I started SS in march 2017 after 8-10 years of lifting aimlessly for exercise. I'm 5'7" and my weight has gone up from 180 to 188. I started my squat in march at 225 3x5. I have been having some low back pain on my left side that I believe is SI joint. I feel it while squatting but it doesn't get worse and isn't excruciating. As the weight has gotten heavier I have noticed my hips shift slightly to the left on the ascent. I am posting 2 videos from 03/28/17 at 310 3x5. Rear oblique was my second set and rear was my 3rd.
On my rear oblique set I felt my knees caving so I narrowed my stance mid set.

Rear-
Squat Form rear - YouTube

Rear oblique-
Squat Form rear oblique - YouTube

[Tom Campitelli]

Yeah, you have a little shift going on in the squat. Could be a slight leg length discrepancy, although I am not really convinced of that. The big thing you need to do is drive your knees out harder and do it much earlier. That will probably tighten things up a bit and may help to alleviate the shift. Work hard to push evenly with both feet on the way up.