Physical damage to the drainage field / soakaway.  In some cases a septic tank can flood due to the receiving soakaway pipe work becoming damaged and broken.  This can often be due to heavy machinery moving over the soakaway and crushing the pipes.  Older ‘but-jointing’ soakaway systems were constructed using clay wear pipes and it is common for these clay wear pipes to become dislodged due to ground movement, resulting in soil entering the pipe work and blocking the soakaway.