Exposure... Either long-term exposure to the elements (sun, water, freeze-thaw) or shorter-term exposure to damaging air pollutants, and chemicals
Structural Movement... Such as building settlement or expansion/contraction not accommodated by the roofing system
Biological Growth... Such as vegetation in areas of standing water or algae
Not Fixing Problems Promptly... These can add up to a much shorter roof life--e.g., if a small problem is not repaired, then a large amount of insulation can be damaged
Forgetting About Maintenance... This is perhaps the biggest cause of premature roof failure
Change In The Use Of The Building... e.g., an increase in the interior relative humidity of a building can cause severe condensation problems within the roofing system