Most newbs have a tendency to let their hips come up in both lifts.
In other words, I would wager that if you were to video your lift from the side, when you come out of the hole in your squat, or when you break the floor on your pulls, your hips are coming up before the bar moves, and this places stress on the low back.
We always try to teach what I would term patience when deadlifting. This is not at the expense of aggression. What I mean by this is keep your hips down and breaking the floor with your leg drive, not the low back. Don't break the floor with your low back, sit back and use your legs.
The reason I call this patience is because sometimes, particularly in a sumo pull, this can make the lifter feel like breaking the floor is taking forever. This is really just your brain slowing down time. What feels like a couple seconds is actually tenths.
Both lifts start at the top. Start the lift the way you want to finish. Make sure you are locking your hips and knees at the beginning, and focus on keeping your chest up as you descend. Big breath of air and push your belly out.
When coming out of the hole in a squat, think about thrusting your belly forward. Ideally, the movement in your hips should be forward, not back and up