7 1/4 why weak?

I was talking to a guy that put a 440 into a dart, the motor had some work done to it, the car still had the 7 1/4 open rear end in it. He was about to put an 8 3/4 in but wanted to take it for a spin to see how the motor ran, was cruising and gave it some gas, well the rear didnt really like that and decided to completely blow apart. Didnt wreck the motor or trans just stripped everything out inside the differential like it was made of butter lol.