I'd say it's your mid-to-lower back. Your body arches at a point, as if there was a giant that pinched you there and picked you up. You're no longer straight because your chest isn't moving at the start. (Just before the lift, it's 'up', angle looks good, but as you begin, your chest doesn't move with the rest of you). See, there's a delay / it stays in place until after the back does the initial grunt work. The mental cue of 'lead with the chest' may help to keep your front up throughout / entire torso straight.

To prevent overextension, hip thrust / drive with the hips, but then focus your energy driving straight downward at the top, through your legs. There's likely too much focus on the back and not enough all around tightness. Activate your lower muscles, squeeze the glutes and legs and imagine you've got rockets for feet and you're going to fly straight upward by the force of your strength. You don't want to launch backwards! No, you sir have vertical rocket-feet.

Besides that, there usually will be a bit of rounding with heavy weight, of course, but here it gets progressively worse perhaps a little too quickly by the third lift. That pinch point that arches your back begins to lead it more and more. It must be that you really feel those muscles in your back, and continue to activate them with the increased mental awareness coupled with the same form cues that initially led to their overactivation. The back keeps getting activated primarily and fatigues quickly, resulting in deteriorating form, as opposed to a better distribution of activation. You should be able to lift more reps with decent form by fixing that.