US elites got used to a very sweet state of things in transatlantic relations post-1989, and it got even better after 9/11. Steamrolling Europeans was almost too easy, because Euroelites had been fully co-opted by the American way.
Unfortunately, 13 years of unpopular wars have taken their toll. There are now two generations of politically-active Europeans who were not invested in the Cold War, and see the US fundamentally as a big bully. Couple that with tight economic times (which, many feel, have been foisted upon us by reckless American finance), waves of unwanted migrants generated by US foreign policy, and the self-inflicted loss of influence of "Airstrip One" Britain over EU matters, and you have a situation where US negotiators likely found themselves surprised by European pushback on all sorts of topics.