The common wisdom with any team going into the playoffs is that you don't want to peak too early. Many factors can contribute into this: injury, chemistry, strength of schedule and just plan luck. If players were robots (or at least cyborgs, like the Terminator), then the mental aspect of momentum wouldn't come into play -- have a bad game, shake it off and treat the next game as brand new.
However, we're dealing with real people here with varying degrees of mental toughness. Team history and personal history (and personal demons) can creep into one's psyche, causing doubt. We've seen it all too many times -- when a player or team's confidence drops, everything falls apart, but one good break and that belief comes back. The legs pump harder, the shots seem crisper, the timing gets better.