I think it's a mix of things. Some of these asshats genuinely believe they are in 1933 Germany and that Trump is planning to let loose a brownshirt army. Somebody ought to tell them that Trump just pulled an epic con job on the racist elements within the GOP, and he has no use for them anymore. Some are horrified that Trump will undo the Paris climate accords. This is probably going to happen, but any GOP president would have done this after taking donations from the fossil fuel companies - Trump is not unique in this regard. Others are worried about Trump's proposals on immigration, healthcare etc but again, Trump isn't going to follow through on the BS he spouted during the primaries.
Bottom line, it's an overreaction.