Ya, it's just your ingrained habit at this point. And those can be tough to break.
It's not that it's "wrong," it's that it's not the most efficient way to pull a loaded barbell off the floor, and will ultimately put a lower ceiling on the weight on the bar, and you're making things artificially harder than they need to be. If you notice, there is, in fact, some horizontal movement of the bar right off the floor. Watch the plates when you drop your hips - they roll forward a bit. When you pull off the floor, the bar swings back towards you right at the beginning. It's not a lot of horizontal deviation, but it's some. You can get away with this because it's 265, a relatively easy weight for you. But what you lift this way will be some amount less than what you could lift if you did it efficiently.
It's not that you can't get into extension without dropping your hips. It's that you're not very good at it, because you haven't been doing it. To get good at it, and in the long run lift more weight, you're going to have to take weight off the bar to the amount you can lift with keeping your back flat. And work your way up from there, bit by bit. The last rep in your first set isn't TOO far off the mark. In fact, in your second set, I don't think you're setting up correctly either. I can't see your feet and shins, but the bar still does swing back a little. Are your shins in contact with the bar when you set up? I'm not sure they are. Hip height is set by the point where your shins contact the bar. Not lower, but also not higher. Shins must be in contact with the bar for a straight bar path, and also to ensure you're able to use your quads to break the bar off the floor, not lever it up with your back.
If you get the bar, and your shins in the right place, squeeze your back into a hard extension, and initiate the pull by straightening out your knees ("push your feet into the floor") while keeping back flat, I suspect you'll find an easier time keeping in extension.
Last edited by Michael Wolf; 08-07-2014 at 09:01 AM.