Backflow may occur simply because the public water system lost pressure. Backflow, reversal of flow from its normal direction, is usually caused by a back pressure or backsiphonage. It is a condition that manifests itself when the water pressure within an establishment’s plumbing system exceeds that of the water distribution system supplying it. This back pressure might be caused by a difference in elevation, a pump, a steam boiler or other means.
Back pressure or Backsiphonage may occur when the water pressure within the distribution system falls below that of the plumbing system it is supplying. This may happen due to a fire department pumper truck as it needs to pump water out of the distribution system faster than the water treatment plant equipment can replace it. Also, the water rushing downhill due to a broken water main might create a partial vacuum on some plumbing systems connected in the vicinity of the break and cause a backsiphonage or, perhaps, simply flushing the water pipes to clean them may cause this phenomenon.