IMHO, bridges sometimes just crack. There were some balls a few years ago that were particularly prone to it, but that doesn't seem the case these days. Whenever you put 2 x almost 1" holes right next to each other, you're dramatically weakening the structure of the shell. However, if you made the shell strong enough, I bet nobody could hook it these days. Urethane balls last almost forever, for example.
Added to the fact that the modern game of throwing the ball almost straight and letting the ball company engineers hook it back means that players track really high these days. This means that the bridge is likely to strike pins, which act as big white hammers on the ball. This is purely anecdotal eveidence, but I'm sure I see a lot more bridges crack on guys who track really high. Especially as balls age and the bridges soak up oil, as it can wick down the sides of the grips and penetrate the coverstock from three sides of the bridge.
Today's snow tyre ball covers just don't have the structure to last as long, which of course, is just great if you're a ball manufacturer, as you want them to wear out after lasting just long enough. People often complain about how balls get so much more damage than the old days. I say the reason lies with the balls much more than the centre operators in most cases. Players prefer gear to hook more rather than get off their dates and hook it, so manufacturers are happy to oblige, but it comes at the price of ball longevity.
As for balls that crack, I'd rather talk about the good ones. But that's another thread.