It had little to do with Hitler specifically, but rather the fact the Americans were at war with Germany in the World Wars. You can also see evidence of prejudice towards Germans in America during and after the American Revolution because of the Hessian and other German mercenaries fighting for the British, despite the fact that several Continental Army Generals were Prussian, and introduced military drills which was important to their victory over the English.
Yes because of all the bad things that hey did in the First and Second World War under Hitlers reign! It was inevitable to happen because Germany was universally detested for what they did!!!