Good point.
Also, (and this goes for all big exercises, especially compound lower body movements) once the primary movers fatigue and the set is carried on compensations via other muscles occur.
In the case of deadlifts, the glutes will fatigue first and the ham strings will start compensating for the glutes. As these begin to fatigue the spine begins to round as the spinal erectors (which normally do little dynamic contraction) start shortening to extend the back. This is why you're back is sore. This is also not good (terrible) for the spine.
Stick to lower reps, and each rep should almost be treated as a separate set. Drop weight, reset grip, reset hips and shoulders, pull.
Br