But most are reasonable fears, or were at some time.
Friday is considered a day of bad luck in some cultures. 13 is an Imperfect number (the perfect being 12; 12 greek gods, 12 months of the year, 12 hours in a day and night, etc.). Imperfect things are considered the Devil's work. Thus, Friday the 13th is considered an unlucky day.
In Europe, cats were considered animals of witches, or a witch in disguise, and the Devil's helpers. So, they burned cats. The colour black represenst desease and death. Also, black animals, such as crows, are believed to be soul carriers in many Native American cultures. So, black cats were witches who stole your soul. Thus, the fear of black cats.
Throwing salt over your shoulder was used to "blind" demons.
When you walk under a ladder, it's very shaky. So, walking under one was pressing your luck. It also has something to do with triangles, but I forget what that's about.
If you think about it hard enough, you can think of perfectly reasonable reasons for these fears.