Well, I'm not a massive string breaker like some of the guys I play with. There are a number of factors that break strings:
1) Swing speed. Obviously the more energy you impart into the ball, the more general wear on a string.
2) String movement. If the face has an open pattern the strings move. this causes friction on the strings that causes fraying in something like a multi and a natural gut but also causes stretching which can cause the string to snap if the elasticity threshold is surpassed.
3) Off center hits. Hits to the outside of the hoop can cause sheer forces against the grommets and knots along with there being less strings to take the impact.
4) Form. Good technique often increases string breakage. Very good technique causes the most energy transfer per swing. This is a bit tricky to describe. Its best illustrated in the serve. A good server gets his body moving up into the ball via his legs, his arm swinging and hitting the ball at the peak of its rotation, and his wrist snapping down all exactly at the correct time to impart ALL that energy into the ball. If any of these are out of phase there is wasted energy that does not make it into the ball.
I break strings mostly because of #2, #3, and to a lesser part #1. I normally don't break them from off center hits, but the 99s seems the be particularly sensitive to this with gut and sheer forces causing a break. I do swing fairly fast with a semi to full western grip.