It's like a team that re-discovers why they're playing the game, and they see that their effort and exertion are causing the cheering in the stands, which only makes them work harder. They stop worrying about losing and focus on playing harder, and on each subsequent play they are playing harder than their opponent.
The same is true of communities. Few people like to be the only contributor, the only person pushing to make something successful. Showing momentum is crucial to build to critical mass, whether it's the user base, customer base, or contributor base for a project.