There are two main causes of backflow. These are known as backsiphonage and backpressure.
Backpressure
Backpressure is a situation that can arise when water from a non-potable water supply becomes greater than the pressure of your main water supply. This can cause dirty or polluted water to flow back into your potable water system and contaminate it. Backpressure can also be caused by a blocked drain, which will prevent normal drainage and create excessive backpressure.
Backsiphonage
Backsiphonage is the process of water being forced back up into a home or building’s water supply line due to higher water pressure in the main supply line. This can happen when there is a break in the main supply line, or when there is an excessive demand on the system (such as during a fire). Backsiphonage can contaminate the water supply with bacteria and other pollutants and can cause significant damage to plumbing fixtures and appliances.