Struts do three basic things; they act as an upper control arm, support the car's weight with a coil spring and control wheel movement with a shock absorber. Strut failures can happen in a number of different ways, most related directly to time and wear.
All struts wear out over time. Spring material is just like a paperclip; if you bend it back and forth fast enough and often enough, it'll heat up. Repeated heating and cooling cycles affect the temper of the springs, causing them to soften and sag under the weight of the car. These same cycles will also cause the oil in the shock absorbers to break down, getting thinner and reducing the shock's ability to dampen movement.
Aggressive driving and off-roading will subject the strut springs and shock oil to more extreme heat cycling, which will cause them to prematurely wear out. Larger-diameter springs, higher-capacity shock absorbers and external oil reservoirs can help to prolong strut life, but won't make them bulletproof under extreme use.
A heavy wheel and tire combination will put more stress on the strut assembly, particularly the shock absorber. While even a slight increase may shorten strut life, a very heavy off-road tire will really kill the assembly. If you're going with a wheel-and-tire combo more than 10 percent heavier than stock, then upgrade to a heavy-duty shock absorber.
