Many years ago, people believed that the good spirits lived on the right side and the bad spirits lived on the left side.  If someone spilled salt, he/she was sure his guardian spirit was sending him a sign by spilling the salt to warn him of evil close by. 

At the time, salt was scarce and hard to find.  To prevent the evil spirits from harming him or her, he or she would quickly throw a pinch of salt over his or her left shoulder.