I, a Roman Catholic, believe there is a demonic influence involved, but I also believe that there is a psychological effect as well, and that the causes will vary to cause confusion.
Now the non-paranormal cause of the Ouija Board is called the ideomotor effect. First there's anticipation of something happening, then there's the subconscious thought of the answer you secretly want. You subconsciously think "Yes" answer, then your body follows that though and moves to Yes.
It's even more seemingly incredible when it is done in a group, everyone's hands are moving, which heightens the bypassing of your awareness, and you really think there's something spiritual at work.
There is a reasonable explanation for atheists to give for the Ouija Board, but that doesn't mean demons have nothing to do with it, they just won't always play, thereby allowing others to doubt and come up with atheist answers.
SO REMEMBER: IDEOMOTOR EFFECT.