Continuity mistake: In the rooftop scene, Buffy's hair is partly tied back and slightly curly but by the time she reaches the Police station it is completely loose and much curlier.
Revealing mistake: When the insect creature is crawling on the ceiling its hair doesn't falls down towards the floor, it "falls" towards the ceiling. (00:22:05)
Answer: Due to his epiphany, Angel basically had the 'true darkness' he worked so hard to reattain ripped away from him, like his soul was after sleeping with Buffy.