Video would confirm, but I bet you are losing thoracic extension.
I don't know why this is happening but the bar rolls up my back around rep 3-4 and makes it really challenging to stay balanced and keep squatting. Unfortunately the camera cut out during my 195 work sets but I have a clip of a warm up at 135x2. Maybe that shows the issue. This has happened the past two workouts and it made me fail a rep today because it rolled onto my neck and I proceeded to squat high bar and lose my balance because of my forward lean. Last workout I was chalked thoroughly, today I was not. I'm ticked because reps 1-3 are really strong and controlled but then 4-5 are really difficult with the bar smashing my shoulders and rolling all over the place. I will be posting the warm up set in less than 5 minutes right here. Thanks for the help.
Video would confirm, but I bet you are losing thoracic extension.
It is shot at a front 45. Would that work for identifying the flaw? It's uploading to YouTube right now.
Showing it from an angle where no one can see anything should be great.
I was guessing that perhaps your hands were under the bar rather than behind and that maybe you were therefore pushing the bar up as you leaned over, but, although it was hard to tell, it looks like your wrists are straight with hands behind the bar.
I think we will need an angle from the rear, but to my inexperienced eye, it looks like the bar may be a bit too high.
Build up your Traps.