• There are two kinds of shock boots. One protects the seal between the shock and the fork tube; the other protects the fork seal and the entire exposed (chrome) area of the fork tube. The later is much easier to tear because it's larger and more exposed. While damage to "accordion"-type full covers is more common, the causes of tears in both are the same.
  
  

Age

• "Accordion"- or gaiter-style shock boots have all but disappeared in the last twenty years. When you encounter them, they're probably old. They're rubber. To function, they have to flex. Age will dry them out, then use will crack and tear them. The smaller fork seal type of shock boot is also most likely to tear because of age.
  
  

Exposure

• Like age, exposure dries rubber. If your motorcycle is outside where it is exposed to UV rays and cycles of heat and cold, wet and dry, freezing and thawing, the rubber is going to dry and crack. Once it's dried and cracked almost any contact or movement can tear it.
  
  

Debris

• Dirt bikes, especially dirt bikes with full shock boot covers, are much more likely to have torn shock boots from debris than street bikes. If you're riding with other people their wheel spin can throw gravel at you (a primary reason for having full shock boots) and your motorcycle. It's not likely that gravel thrown from another bike, by itself, will tear your shock boot. But with even a small amount of dryness or cracking from age or exposure, this can happen.
  
  

Accidents

• Of course, accidents can damage every part of a shock boot. But dirt bikes are especially vulnerable to shock boot damage in an accident for a couple of reasons. One, people crash dirt bikes more often than other types. Dirt bikes are also more likely to fall in an area where there's brush or protruding branches that may tear the shock boots.
  
  

Corrosives

• Where you ride matters. If you're riding a street bike where chemical deicers are used, they can damage rubber parts. (That scenario sounds unlikely, but it exists in mountainous areas where there's a lot of sun: it can be icy in the morning and 70 degrees at noon.) A dirt bike is also likely to encounter agricultural corrosives, including cow manure. Any corrosive you encounter can weaken all rubber parts, making them susceptible to tearing.