Fear is an evolutionary skill that has helped people survive. Fear is why you look both ways before you cross the street, why you do not try to stick your hand on a hot burner and what makes you suspicious or untrusting of those who threaten your status in life, whether that is financially, romantically or physically.
It makes sense then that many people would feel fear in response to a new potential threat, like thinking machines who could potentially infiltrate their daily lives. If AI machines began learning and developing on their own, scientists are concerned that they could surpass human understanding.