Since I've been on the topic of The Shining lately, Ken Hanke at mountainx.com has a truly insightful review of The Shining. He does a good job comparing the book to the movie and showing their strengths. Of course, he wants to read the book as a psycological thriller, and this leads him to a problem:
"The problem with reading The Shining as purely psychological is that it raises the question—in terms of the action—of who (or what) lets Jack out of the pantry after Wendy imprisons him there. I’ve heard the argument that Wendy herself perhaps let him out, but that seems off-base to me, not in the least because at that point she has yet to experience anything inexplicable. When she finally does see the Overlook for what it presumably is, she sees—and hears (there’s an indecipherable chanting on the sound track that might well be some black-magic ritual)—more than Danny or Jack ever did. It’s as if the evil has to completely reveal itself in order to penetrate her less suggestible mind."
Simply put. . . I think the ghost did it.