The 1953 coup [1] probably has something to do with that, followed by the hostage crisis in 1979 [2], and the downing of an Iranian passenger airplane by the US [3].
1. https://en.wikipedia.org/wiki/1953_Iranian_coup_d%27%C3%A9ta... 2. https://en.wikipedia.org/wiki/Iran_hostage_crisis 3. https://en.wikipedia.org/wiki/Iran_Air_Flight_655