One simple answer is it is a mindset. The Colorado Rockies are still a fringe candidate for the playoffs (as of today they sit 7.5 games out of the second Wild Card). When playing a good team, the motivation is there to play and perform well to show the opposition what you can do.
For example, the Rockies are 2-1 against the Baltimore Orioles, 4-2 against the Chicago Cubs, 6-1 against the New York Mets, 6-7 against the San Francisco Giants and 2-1 against the Nationals this season. All of these teams have winning records.
However, when playing the Phillies and Brewers, whether the Rockies thought they could simply show up and win is an interesting theory.