Superstitious behavior arises when the delivery of a reinforcer or punisher occurs close together in time (temporal contiguity) with an independent behavior. Therefore, the behavior is accidentally reinforced or punished, increasing the likelihood of that behavior occurring again.
For example, you walk under a ladder and a minute later you trip and fall. It is easy to attribute your accident to "bad luck" and the irrelevant ladder. The reason an association is easy to form is because your cultural belief that walking under a ladder will bring bad luck is positively reinforced by your fall that occurred soon after walking under the ladder.