Here is an example: A person is stung by a bee at an early age. Immediately the reactive mind begins to record everything that is going on. The color of the bee, the feeling of being stung, the disappointment and crying, his mother's voice saying "What happened?" and other things going at the time are all recorded.
Later on in life, whenever that person sees a bee he feels sad, afraid and is reminded of the pain of being stung. It is caused by the reactive mind. As are many other irrational phobias.