Poor Maintenance:
Systems that are not pumped on a regular basis can build up an excess of sludge within the septic tank that can then pass into the absorption field forming clogs that can not be removed.  Flushing household chemicals, oils and other substances can also damage the system.    
Overload:
Systems are designed based on the number of bedrooms in a home at the time of installation. If the household grows larger than its initial occupancy or is overused for any number of reasons, the system may not be able to handle the increased water load.  Irrigation systems can also add excess water into the system.
Improper Design:
Septic fields that are improperly located or graded improperly can cause wastewater to be unevenly distributed.
Physical Damage:
Driving, paving, or building on top of a septic system can damage it.  Heavy equipment can compact the soil preventing it from absorbing at the required rate and sometimes, not at all.  Building on top of a system can prevent future access for maintenance and repairs.
Soil Conditions:
Soil around the drain field may be too wet, spongy, or saturated with storm water or rising water levels
in nearby lakes, rivers and ponds.  In warmer months or during dry periods, the soil above the
absorption field trenches may not have enough water-holding capacity, causing the grass to turn brown.  
Roots:
Roots from trees and shrubbery adjacent to or even on top of a septic tank and or drain field will
eventually find their way into the system.  While roots can be removed from inside of the septic tank in
most cases, they can not be removed from the drain field.