Your back is very rounded from the mid back down resulting in "catback" when pulling.
I would try to get your hips down a little bit lower before starting the pull, setting your upper back harder and not taking so long from the time you grab the bar to the time when you pull.
There is also some hip shifting/rotation going on. Can't really assess that from this video.
Granted this was a 1rm things are going to go to shit, but I'd recommend trying to fix some postural issues at around 80-85%