Why ask an American this question, not a Pakistani?
Still, as I see it the process started during the 1965 war with India. Z.A.B. encouraged Ayub to start the conflict and tried to convert Pakistan's mutual defense agreement with the U.S. into an alliance against India. The U.S. refused.
Z.A.B. blamed the U.S. not his own over-reaching ambition. Most everything worked from the top-down in Pakistan then. The U.S. continued to support Pakistan for reasons of geopolitics and good faith even when Bhutto was in power, even though Bhutto continued to publicly blame America for the bad results of his own policies, so neither Pakistan nor Bhutto
lost anything. Z.A.B.'s anti-American slowly permeated the Establishment and eventually the schools - and after Zia came to power, the madrassas.
However, there is also
Christopher Hitchens' hypothesis: