App crashes take place mostly because of the following reasons:
Hardware problems like camera or GPS services
Problems in internet connectivity like sudden connection failures
Memory issues: App trying to access memory location for which it does not have permissions
Trying to execute invalid or unauthorized instructions
Analytics and advertising systems used by developers can also cause malfunctions.
So it's clear that crashes happen due to a mixture of hardware and software related issues. As newer versions of an operating system are released, developers have a daunting task of testing their developed application on all versions before sending it in the market.
Running OS 10.7.5... my Preview app's going bonky on me.... keeps crashing, regardless of what kind of document I'm trying to open with it. I dumped preview prefs, rebuilt directory, repaired permissions, etc.