There are two situations that could cause backflow and they are: backsiphonage and backpressure
When there is a sudden reduction in water pressure in the public drinking water distribution system, such as during fire fighting or when a water main breaks, water flow can potentially be reversed. This could create a suction effect, possibly drawing contaminated water into the drinking water system.
Backpressure is created when the pressure in a private non-drinking water system, such as in a recirculating system containing soap, acid or antifreeze exceeds the pressure in the public drinking water system that it is connected to. This could force contaminated water to enter the public drinking water system.