The nervous system is prewired to some set of associations, but not others. This fact led to the development of concept of “preparedness,” which accounts for certain complex behaviours.
For example, when two unrelated rats are kept in a small box and subjected to a mild shock by electricity, they fight with each other although neither of the rat is responsible shock in fact. This experience make the brain to associate injury with other animals or unknown objects.
Thus, when two unrelated events occur simultaneously and experienced together, both become inadvertently associated. For example, an unexpected pain in the presence of unknown person lead to a faulty association between the pain and stranger.