I was in at the gym at my college the other day putting my girlfriend through a workout. She weighs about 135 and was doing 95lb deadlifts. The WOD was scaled down from one I saw from the exercise demos, 10 DL and 10 push-ups 10 rounds for time. She is a college athlete (swimmer) and I made the assumption that she was proficient at the deadlift. After lifting the weight several times her back began to round, bad. We dropped the weight down by 20lbs and she did several more reps and the same thing happened. We dropped it down again so she was lifting 65lbs and her back was still rounding. After the workout we did a couple of reps with the dow and she was able to maintain her lumbar curve and hold a consistent back angle until after the weight passed her knees. She went back to the bar and tried it and her back rounded out again. I read Ripptoe's article on the analysis of the deadlift and I think it could be either her hamstrings and glutes aren't strong enough to hold her hips down or her lower back isn't strong enough to lock her spine in ideal position.