In reply to HP:
apparently that has to do because yesterday, when some of his teammates were involved in a crash, he 'neutralized' the peloton, meaning that as the leader of the race, he slowed down, so no attacks from others etc... . That's usually only done when a team leader is involved in a crash or has a mechanical problem, but it's considered bad style to do that in order not to be separated from your team mates. well, I didn't watch the race on thuersday, but that's what I heard from an expert. if all the spectators booing did it because of that, no idea...
That neutralizing of the race by a main protagonist happened when Jan Ullrich fell in a the descent from a col, Armstrong slowed down to allow Ullrich to catch up. Once that done, the attacks started again.
An other famous exemple, but bad style, is, when Contador attacked A. Schleck on a col just in the moment he noticed that Schleck had troubles with the chain/gear