Okay, so the other day I was thinking about one episode of Doctor Who, where the Daleks shift a part of the universe a second forward in time, resulting in a pocket universe. So I thought "Hey, what if StarClan is like that?" So I give you this theory. Enjoy.
StarClan may be shifted forward a second in time, in a pocket universe. When it was shifted, certain parts of the area became extremely unstable and collapsed, resulting in dimensional rifts connecting to the nearest universe. The normal Warriors universe is the only nearby universe, so that's how StarClan communicates with the normal cats. This may also explain why it is almost always a certain time and how certain areas are off-limits to every one. If they are actually off-limits, they could have become superdense during the shift, resulting in a thick, unbreakable wall of air molecules.
Any questions? :P