I really thought this video was sped up. Slow. Down. You lose back tightness because you never get tight in the first place, or if you do, you lose it entirely by collapsing down so quickly (falling = something has to loosen). All of this is very likely the source of your aches and pains (I had the same problem of dive bombing my squats so I can testify personally here).
Your stance width from toe to toe looks good though I'd narrow the distance from heel to heel.