Just anxiety, no true way unless its a near death situation or death of loved one. Such as say a son is stolen in a crowded mall, the father could get a phobia of crowds.
That's a complex question to answer but in simplified terms it comes from internalized learned fears from life experience and innate survival instincts. For example, we're essentially large sacks of fluid so it's natural that people would be afraid of heights.
Semi-Insecurity/fear complex's ***.
Lately? EP. Some people freak me the **** out on here.