It wasn't like he was a fan before, of US and west, clash was coming either way. But what sparked his enemity was after Iraq and Saddam threatening to invade the Saudis, the royal family turned to the Americans for aid and defense. Having Americans troops on the Holy Land to defend it ( the oil) was pretty much the ultimate insult to him and zealots like him. A defenite sign of Muslim oppression changing from East to Western fist.
Changed goal from a more simple Mujahideen supporting fellow Muslims on regular battle grounds (who the US found it smart to excessively encourage to become more extremist Vs the Soviets who they said hates Muslims, litterally helped them gain power across Muslim world instead of Arab secular groups that were less likely to hate soviets...) to bringing the fight to the US in irregular fear tactics of terrorism, from Yemen to attacks in East African targets (couple distant relatives who died there), to the ultimate attack that seems to made people think they sprung out from nowhere out of the blue. When they obviously didn't.