Similar thread topic here and the info applies to you also.
Fatigue for sho could contribute. With high rep deadlifts it’s not uncommon for the back to round out. If you use a belt properly it should only help. If you are using a belt improperly or train mostly beltless it could help little to none or even make things worse e.g. if you hinge/bend at your belt/lower back.
Aside from the lower back rounding / snap city action I’m seeing that you’re having difficulty keeping the bar close to your body and there’s significant windmilling (where the bar kind of spins/rotates with the underhand end of the bar being pushed away from your body often due to using a funky mixed grip). Probably contributes to the back rounding and in it’s own right is an error which requires attention.