Hi Jack,
1) Slow down both your setup and your back flattening a bit.
2) I would get your setup in place first and make it distinctly separate (wait a second) from the back squeeze. When you set up, try and keep your head more neutral (look about 6-10 feet ahead on the ground).
3) You are squeezing your back flat, but squeezing the slack out isn't really happening at the same time. It's delayed and this causes you to lose the flatness as you pull. Try and pull the slack out as your flatten your back. Then slowly push the floor away (stand up) to start the rep.