Try loading the bar, by getting tight and starting the pull as you're getting set up. This takes the slack out of the bar and when you really start to pull, the bar will move smoothly off the ground. You're sort of jerking it off the ground to start, which is allowing your hips to rise up.
When I set up to pull sumo, I get my feet set, then with locked knees I lean over and grab the bar, get my entire back arched and locked into position, then pull my hips back and down while pulling against the bar. With weight under 315, the bar comes off the ground before I'm even fully in position to start the pull.