What you're seeing is your back come undone. You could do with setting the back harder and be more deliberate about that step before you break it off the ground.
265x5x1 deadlift. Any form critique would be appreciated. I’m not sure if I dropped my hips on the last 2 reps, but my hips still seem to rise before the plates leave the floor and it looks like my knees extend early. So I guess my question is, are my quads not strong enough to break it off the ground in that position, or is my setup just wrong? Deadlift 265 - YouTube
What you're seeing is your back come undone. You could do with setting the back harder and be more deliberate about that step before you break it off the ground.
This happened to me on my set of 335 last week. Not cool...