Causes of broken ball joints

[Sixball]

This is the second time on TWO different trucks I've broken a ball joint and left both trucks sitting on the frame.

First was my 94 GMC 1/2 ton stock height. It was my daily driver. Mostly in town. Steering got stiff right after new tires were mounted. Drove it 10 miles, and luckily in a parking lot at 5 MPG it snapped (drivers side) and shoved the wheel up under the fender.

Second time was on my 1995 Silverado 1/2 Ton that is lowered 4 inches. In my driveway at 3 MPH, it snappec and shoved the front tire up the fender (driver side). Kinda scary if it happened at speed. What causes this ?