Posted by Anshul: This seems just a minor set up error to me. If you watch closely in all the reps which upper back has not rounded, she was pulling with hips in proper position i.e. almost in midway between shoulders and knees. In the reps in which back rounds, she is pulling more with hips upwards i.e. back stays almost horizontal which is causing the thoracic to round little bit due to weights pulling it down. A little more hips lower like in the last video should be good enough. Doesn't seem a major issue to me. I am sorry to step in here if it may seem. I am no expert though, so I will wait for others to see.
Nail on the head. The only thing I would say different, would be the "hips in proper position and halfway between" part..that could mislead people as there is a slightly different position for everybody and some do need a higher hip position because of body proportions. But anyway, Anshul called it right on. You've changed your set up and brought your hips up. What's funny is you're noticing the "thoracic rounding" but not noticing that you lumbar, which originally manage to stay so set and arched, started to flatten (i.e. round) and then go past flat…now…think! Lumbar rounding is usually accompanied by thoracic rounding. Look DOWN the kinetic chain first!
This is a symptom of having a layoff and coming back in without the proper plan. I don't know how you came back to it, but this should be the goal, in a nutshell, when coming back.
1. Recapture previous maximum strength level AND movement quality (get the groove)
2. Build endurance
So, for instance, if you had previously been at some base volume, then you have a layoff, when you go back in and start right off trying to regain that base volume, you haven't first got your groove back, and all that, you can have these hiccups like you've had here.
So, this is a call for some kind of honeymoon/slash quality volume sufficient to get your previous performance and setup nailed down..then continue from there. No serious problems here..nothing that can't be ironed out with some patience and a little time.