Could be abs or lower back, not neccessarily a belt, also you may have been fatigued and your hips came up first and not your head
Using a wide hand placement was suggest and that helped some but I also found that if I really focus on flexing my hips out of the hole I'm able to keep myself more up right during the way up. Of course strengthing your low back and abs is always a good idea too though.
If not a "physical" thing, you may have lost your concentration. There is a lot to think about during a sqaut, and the heavier you are the less room for error.
Did you drop your head maybe? Or did your hips come up before your shoulders?