Let me preface this by saying I may not know my ass from a hole in the ground here, but your back doesn't look too rounded at any point here. The last rep on your third set your neck angle changes on your ascent out of the hole, like you're leading with the top of your head. Maybe it's nothing, but that change in neck angle could have changed where the load is sitting on your back and the muscles supporting the load changed with it (putting a different strain on your back). Again, see the "ass from a hole in the ground" disclaimer though.