Did we talk to you about keeping your hips up at the seminar?
What did we say about Ed Coan and his deadlift? See any similarities?
I think this is happening because you're setting up with the bar about 1/4-1/2" to far forward (hard to tell from over here), based on your knee position and your low hips. Have somebody check to make absolutely sure you're putting the bar over your midfoot. Finally, you're not finishing the lift - your knees and hips are not fully in extension. And get your chest up.
Other than that, a fine effort at 420. Looked pretty easy. Should be even easier after you fix this stuff.