Just on the fairly obvious and widespread obswrvation that nobody is elected in a vaccuum - voters react to politicians that have gone before.
People feel disenfranchised by the current crop of politicians hence there have been anti-establishment votes fron Scotland to Brexit to Trump.
It's easy to overstate the case. The history of American elections shows that since the Second World War and all things being equal, American voters tend to elect one party, re-elect the incumbent for a second term, get fed up and elect the opposition:
1948 Truman (Democrat)
1952 Eisenhower (Republican)
1956 Eisenhower (Republican)
1960 Kennedy (Democrat)]
1964 Johnson (Democrat)
1968 Nixon (Republican
1972 Nixon (Republican)
1976 Carter (Democrat)
1980 Reagan (Republican)
1984 Reagan (Republican)
1988 George Bush (Republican)
1992 Clinton (Democrat)
1996 Clinton (Democrat)
2000 George W Bush (Republican)
2004 George W Bush (Republican)
2008 Barrack Obama (Democrat)
2012 Barrack Obama (Democrat)
2016 Donald Trump (Republican?)