There are many traditions we observe for which we have no reason or explanation. E.g., a person who spills salt will take a pinch of it and throw it over his left shoulder. I Googled it.
Superstitions about salt date back to biblical times when salt was a highly prized commodity. It was expensive, crucial in preserving food, and was often used in place of currency. So spilling salt was considered an almost sacrilegious offence, and left one perilously exposed to the devil’s machinations.
Throwing salt over your shoulder is a way of keeping the devil at bay while you’re in an especially dangerous situation. The salt is either intended to blind the devil so he can’t witness your error, or keep him from sneaking up on you while you’re cleaning up your mess.
The superstition calls for the offender to throw salt over the left shoulder.