I think it has to do with youtube or ISP server load. 1080p needs a decent amount of bandwidth, I know for me youtube has been consitently crapping out lately, where it sits there and thinks whether it should download the video or not for the first 20-30 seconds or not. Then after about 30 seconds of initial download speed (you can see how fast the grey bar is going) it dropps off to about half the speed. I'm fairly sure it's youtube being stupid. Their flash player has been buggy as hell lately too.
it's like why the hell are you bothering to offer 1080p videos when your servers cannot support me wanting to watch them???