Metal fatigue is often the cause of metal parts failing. This is a gradual process and will result in a sudden break.
I owned a Plymouth once with torsion bar front suspension. Both failed; one while I was having lunch and looked out the window to see the car’s front suddenly sag!
I also owned a 1976 Ford Granada with rear leaf springs. Both broke during the time I owned the car.
These things should not happen of course, but are the result of poor quality control or bad design.