Franklin, what exactly are you doing with your pelvic angle during each rep? You're breaking this movement up into unusual chunks, and I
think I'm spotting something going on with you changing your pelvic tilt, and that may be what's initiating the distinct, choppy movement at the top. It's hard to tell with the dark clothing. You're not thinking of tightening your abs specifically, are you?
If you are doing what I think I see, then it'll be affecting your lower back - check out this video, and see if it helps:
https://www.youtube.com/watch?v=NZuB4t5cYkk
You want to get to where the movement starts and ends together, and is a smooth transition down and back up, while keeping the lower back in the same extension the whole time.