ford mustang axle question
I would say maybe a casting flaw. I see stock mustangs at the drag strip all the time with slicks and stock axles and rarely do they have a problem.
The guy I bought my 66 belvedere from said when the car was 6 months old he was driving down the road when a torsion bar broke. He installed another torsion bar and 43 years later it is still holding up.
Sometime these things happen.