Her whole attitude was not perceived as a result of disappointement, she looked arrogant and I guess she really was arrogant.
There's a big difference between her and another American gymnast at the 2012 Olympics: Gabrielle Douglas.
Kayla was arrogant, or at least she seemed so. The result: 1 gold medal (with the team, but she only competed in the vault, Douglas and Wieber did most of the work) and 1 silver.
Gabrielle was humble and sweet. The result: 2 gold medals, one of them as the best all-around gymnast, which is by far the most important medal in the contest.
But I agree with Lips, Maroney was under a lot of pressure given her previous results, and pressure is a tough thing to handle at that age.