(1) The UNDERLYING Reason:
Many are protesting the fear of what will occur in America as a result of the Trump presidency. They contend that their rights will be taken away. They also fear what will happen to non-Caucasians, women, the LGBT community, & other marginalized groups.
(2) UNNERVED, even DISCOMBOBULATED at Election Outcomes:
They are quite surprised, even unnerved that Trump has won the election. They wanted Clinton to win. They viewed Clinton as a symbol of an overgrowing, evolving America & saw the Trump win as a regression, even devolvement. They refuse to even consider how Trump will improve America, concentrating on the economy, not so much as sociopolitical programs. They saw NO WRONG w/Clinton while seeing NO RIGHT w/Trump.
(3) ENTITLED, SPOILED, & IMMATURE Behavior:
They refuse to accept Trump as the new president as many did when Obama became president. When Obama became president, many became quite unhinged- being threatened by the first Black president as such disturbed their Caucasian hegemony. Now having digressed, let's return to the context of the discussion, many AREN'T about to accept the fact that Trump is president. So instead of accepting that Clinton didn't win graciously & maturely, they elected to demonstrate verbally, even physically that Trump is NEVER going to be their president & will do their best to disrespect him at every turn.
So they believe that Clinton SHOULD HAVE won for one reason or another, not realizing that everyone doesn't always get what h/she wants. They refuse to acknowledge that the majority of Americans have spoken & didn't want Clinton whose administration will be a continuation of Obama. The American people wanted a workable America w/sound socioeconomic solutions. However, those who voted for Clinton didn't see this, only concentrating on negative hyperbole so they felt that they were left w/no other recourse but to make their anti-Trump angst VERY PUBLIC. In other words, since they DIDN'T have their way, they intend to let OTHERS know this. To be more succinct, they are acting like spoiled, entitled brats who are whining because they didn't get their election wishes.