A septic field will fail when the volume of water discharged to the septic disposal field is greater than the volume of water that is can be absorbed into the soil or evaporated into the air.
The primary causes of leach field failure include:
Improper discharge of solids from the tank causes the field to clog so it cannot absorb water
Supply lines are clogged or damaged by tree roots, so water cannot be dispersed through the field
Evaporation is prevented by compacting or covering the surface of the leach field
More water is injected into the field than it was designed to handle