Your hip angle is closing after you lift the bar off the floor. What you initially thought was happening is what is happening.
You also deadlift very fast. When you work on fixing your form, you're going to have to slow it down a lot before you can get back to your normal speed.
I generally try to do all of my lifting very quickly, avoiding slow grinding reps. Are you suggesting that I slow it down on purpose? Or will that just be the result of focusing on that part of my form?
While you work on your form, lift at a slow-moderate speed. Once your form is fixed, you can lift at your normal speed again. Deadlifting that fast while you're trying to fix your form will only reinforce your already ingrained motor patterns.
As Donald said you're not tight enough. Get your lats involved, get your shoulders down and back, and think big chest. You can see it gets worse as the weight gets heavier. Some halting deadlifts would definitely help. Start the deadlift and hold it about an inch off the ground for a count of 3, then finish the rep. If you can maintain your position paused you know you're tight.
They may help but the bulk of your work should be using your regular range of motion. Going from a deficit or changing your grip would alter your starting position to some degree and won't be as specific as regular deads or pauses. I would use them sparingly.
Thanks for the help. Would working from a deficit help? Or maybe snatch-grip?
Working from a deficit would help to get your glutes firing properly. In your case, halting deficit deadlifts should be used with light weights. I also recommend the pause that Eduardo mentioned.
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