In plumbing terms backflow is the unwanted reverse flow of water within a water pipe, in a worse case this could be contaminated water mixing with fresh drinking water, causing people to get sick or much worse.
A specialist backflow prevention expert can determine the nature of the hazard that may exist and provide a solution to minimise this risk.
A hose left in a bucket or a tank can allow the content of that bucket or tank to enter your drinkable water give the right circumstances, and you’ll never even know it’s happened until after you are affected by the result. Think of all the nasty thing that could be in a bucket or tank.