In some cases, invisible spirits maybe at work. For those who do not believe in this kind of thing, they may try very hard to find scientific explanations, but in reality, there have been many cases where spiritual solutions were implemented and the problems were solved. For example, mysterious illnesses thought to be caused by nearby evil spirits were cured when the spirits were supposedly sent away.
From what I heard, salt can be a very powerful substance to ward off and repel evil spirits. By throwing salt and lining the doors and windows of your house with salt, evil spirits are thought to not be able to cross and enter your house. This is because of the purity of salt as a substance. Although this may sound like just another superstition, many have used it for years and years or even centuries. This practice can also be seen in the popular TV drama Supernatural.