why is United States hegemoney a Threat?

Answer 1

The US hegemony, or for that matter any hegemony means negation of freedom. One would prefer to be a dissatisfied master than to be a satisfied slave.

Answer 2

Because for hegemoney to be attained you need to embrace the strategy of empire. So other cultures/countries have to continually be subjucated to US control in order for it to work. The power this gives the people directing the strategy continues to corrupt until you wake up to find someone like Hitler or Moussalini in charge. Those who don't learn from history are indeed doomed to repeat it.