Frightening dreams that usually awake the dreamer are caused by what?

Frightening Dreams. Anxiety is the predominant factor behind disturbing dreams. Every dream has a different meaning for every person.
Crazy dreams. Depends on what they are. For example, dreams of blood & violence could relate to bipolar states, dreams of death & loss could mean depression or ptsd, dreams that wake you up in sweat could mean anxiety disorder, etc. Consult a therapist or psychiatrist experienced in dream interpretation.