This is a really helpful look at what the NCAA considers when it looks at granting a sixth year of eligibility: http://grfx.cstv.com/photos/schools/tam/genrel/auto_pdf/comp101-3-five-year-clock-ext.pdf

Basically, a student-athlete has to be looking at missing one or more seasons of competition for reasons beyond their control. That typically means injuries. 

The NCAA actually denies sixth years with some regularity. You can google it and find a variety of examples, especially with football and basketball players.