I have a feeling you might be onto something. It does make sense in a tragic way. Would be very interesting if the overall arc has something major to do with Dawn. Would be a huge conflict considering Buffy was given Dawn to protect, she even died in Dawns place, but what if she was supposed to let Dawn go at the end of season 5. She didn't and now the magical world is bleeding into our world causing causing a build up that will eventually lead to destruction. Can Buffy let go of Dawn? Does Dawn really have to...move on from this world? Lots of possibilities, must wait for answers.