Depends on the situation. You can tell a lot by reading a credit report, but sometimes a LOE (letter of explanation) is needed to fill in the gaps and verify.
A lot of things can happen that are out of an individual's control. Someone mentioned an accident. What about a child or family member (or yourself) requiring medical care that insurance may not cover entirely? What about losing a job...hell an entire industry - as has also already been mentioned? Maybe a spouse runs up a bunch of credit card bills then files for divorce, then a job loss, then a medical catastrophe...shit can add up and sometimes people seem to hit a string of bad luck.
After seeing a lot of BK's on credit reports, then reading LOE's, I've definitely learned not to prematurely judge people for things like this.