Post traumatic stress disorder (PTSD) can happen if you experience or witness a traumatic event, e. g a major accident or military combat. Anxiety, which may come and go, is one of the symptoms. Others include recurring thoughts, memories, images, dreams, or distressing 'flashbacks' of the trauma. It's common to react with anxiety to a frightening experience-it's only considered to be PTSD if the symptoms don't go away. It may develop years after the traumatic event.
Attribution: BUPA