Superstition
Spill Salt, Throw Some Over Your Shoulder

No Use Crying Over Spilt Salt

Another co-worker of mine, coincidentally the youngest of all the people I asked, seemed to have the most insight on the topic. She said that she thinks the reason people do it is because salt was so valuable and important back in the day, that spilling it was seen as a bad sign, because you were wasting it. To this, I asked, “But, if it’s bad luck to spill salt because you’re wasting it, why waste more by throwing some over your shoulder?” She had no answer for this.
She also mentioned that her mother was very aware of this particular superstition and won’t even let them simply throw away the things of salt that you buy, but rather makes them pour it down the drain and dilute it with water if you’re going to do so. She says it has something to do with “putting salt back into the water.”

I read online that the reason people do this is because, like Mayra said, salt was very important in the past, and that “spilling salt was considered an almost sacrilegious offence, and left one perilously exposed to the devil’s machinations. Throwing salt over your shoulder is akin to blessing someone after they’ve sneezed – it’s a way of keeping the devil at bay while you’re in an especially vulnerable moment.”