Mostly in agreement but I also think part of this is the psychology of the opposition. Teams are able to get up emotionally because they are playing the Boston Celtics, the champions and very... pugnacious champions at that.
But, at the same time, they've seen a couple of teams grab wins over this team. Suddenly the fear that would go with playing against the best is fading. It's the best of both worlds for Celtics opponents - the bonus of playing the best without the fear of being the rest, to put it in quotable format ;-)