Why are people vulnerable to stereotype threat?

Here was the irony we had suspected. What made Mikel's vanguard black students susceptible to stereotype pressure was not weaker academic confidence and skills but stronger academic confidence and skills. Their strengths led them to be identified with school, to care about school and how well they did. But in school, when working in difficult material they understood to be ability diagnostic, they encountered the extra pressure of the stereotype. It wasn't low expectations that made them susceptible to the pressure, then; it was high expectations. 

Claude Steele, Whistling Vivaldi: And Other Clues to How Stereotypes Affect Us (New York: W. W. Norton & Co., 2010), p. 58