What happened is fairly easy to explain. Two separate tracks of political motion converged in 2015 to give us what we have.
Several years ago, we started throwing incumbents out of Congress because we were tired of their corrupt, duplicitous system of Washington politics. The new folks came to Washington truly believing that they were charged with doing the will of their electorate. What we didn't realize was that the old system, while corrupt and duplicitous, worked. The "old boys" network in Congress may have been ugly, but it allowed Congress to function. Deals were made, asses were covered, and shit got done.
The new batch's priority was represent their electorate, which they quickly realized meant the people who elected them: their party. This was particularly influencing for the Republican party, since the Republican electorate had become full of ultra-conservatives, militant Christians, and tea party whackos - all of whom demanded their people in Washington not make so much as a single compromise with the Democrats.
This resulted in a "party over everything" mentality that has ground Washington to a halt.
The other factor is much easier to understand. People have been fed up for a LONG time with the politicians the two parties have been giving us. Americans have essentially been voting for the lesser of two evils for the better part of 50 years now. The huge support for Trump and Sanders, both of whom are seen as party outsiders, shows how much contempt the American public has for the two major parties.
