Why do you think a president's popularity surges during and immediately following a national crisis?
In the context of President Bush in the wake of the September 11 attacks, his credibility rating was so high because of the sense of immediacy in his response to the tragedy. At a time when the public was shattered and deeply wounded by what happened, President Bush provided a sense of control and harness to the complex emotions of the time. While there can be many critiques offered of President Bush, history will judge him favorable for being able to rally public support around him in the days following September 11, 2001. The standing on the rubble of one of the fallen buildings with workers and proclaiming, "I can hear you" along with his affirmation that "the people who knocked these buildings down will hear all of us," endeared the President to the nation. The President was able to enjoy great public support because of his firm decision and the belief that he understood where action needed to be taken. During a national crisis, a sense of doom, mistrust, and fear dominates and Presidents who respond favorably to the moment are the ones who enjoy a great deal of public support. President Bush was able to seize this particular moment in order to bolster support of his presidency. It was not something that remained throughout his tenure, but was there at that moment following the attacks of September 11, 2001.
The accepted answer to this among political scientists is that people want to rally behind a single leader in times of crisis.
When bad things happen, people feel insecure. They want to have someone who, they think, is in charge and can tell them how to get out of the crisis. In our political system, the president is the only one who can play this role. Therefore, people look to him (or someday her) to reassure them and to lead them out of the crisis.
In circumstances like this, the president's popularity tends to surge. People give him the benefit of the doubt as they wait for him to lead us out of the crisis. The president's popularity will tend to remain high for as long as it looks like he actually does have answers.