How come M. Night Shyamalan started making terrible movies?

This is a serious question. Now I haven’t seen Signs, but I noticed how he started out really good with The Sixth Sense, Unbreakable, then the Village was still okay (but not up to par), and then it just went downhill. . .especially with Avatar.

Now, supposedly I read The Happening was supposed to be a B-movie. Is that really true or was that just a hand-waver excuse? Is that really why he directed the actors to act that way? If that is true, then he succeeded in making a B-movie. Because once you think about it, when Spielberg made Raiders, it was supposed to be a B-movie, but if it’s critically acclaimed then it’s not really a B-movie now. . .is it?

Anyway. I was just wondering what people’s thoughts were on him and if there was any particular reason why his movies fumbled along the way. I mean, is it just a natural case of losing steam?