Cuz theirs always that point in the pull where your lower back is compromised. That's nearly always where the injury occurs also sometimes at lock out.
The ironic part is I didn't injure myself on a maximal pull. I did it on my 20th rep with a light weight( 315). Plus I was young and way to eager. I know better now.
20 reps was your problem. Deadlifting wasn't made for high reps IMO. No way you can hold anything like proper form doing 20 reps. 5 is a high rep set for me at anything above 60% of my 1 rep max.
