Weak core and or you are bending back too much. I do my standing strict militaries where i finish over my head, not out in front. My best is 230x5, which i am sure is better than most, but not as good as some, and i have never even had a tiny bit of back pain and i don't wear a belt. Try doing them where when you clear your head you start to finish over it. It will make you much stable.