Diving into the world of psychology we can asume that there are two types of actions: the conscious ones and the unconscious. We usually think of something in particular and then we just do it. In some way we think about it but not too much.
Inside the unconscious category (according to psychologist William B. Carpenter in 1852) there is something called the ideomotor (named by himself). This ideomotor is the ability of the mind to project into involuntary bodily movement when instigated by a suggestion or idea.
This is an explanation to many paranormal phenomenon such as Ouija boards. Although I am not entirely sure. It sounds logical but in my opinion it does not entirely explain many cases.