The whole thing between her and Sanders was really silly. I would say that is what caused it. A lot of division was caused when the party became split between her and Sanders in that weird reality TV show thing they called a "primary" which did heavy damage to the unity of Democrat party voters. A lot of Democrat voters are, unfortunately, very childish, so when the pony they voted for doesn't win, they turn their back on the whole party even if it means the Republicans will win the real election. The big problem was the Bernie Bros took the loss way too personaly, and instead of supporting their party, they bought into all the anti-Clinton propaganda - much of it from the enemy Republican camp. Very little of the arguments were even about policy, it was about popularity, it became extremely stupid; an embarrassment to democracy.
On the other hand, Republicans still backed their party even if their pony didn't win their own reality TV show primary thing.
It was kind of sad to see the US descend into so much stupidity. I have read up extensively on the two parties, and Hillary Clinton and the Democrats were clearly much more qualified than Trump and the Republicans. There's a lot of environmental damage occurring right now because of the US, in addition, US military has been murdering people on orders of Trump in the Middle East as he ramps up more military forces to keep stability down, and militant fanaticism up. The US is becoming an enemy of civilization under Trump. I know not all of the US is at fault, but it seems the enemy has their shit more together than the good guys.
Nowadays, the core of the Republican Party is turning into Pro-Confederate white nationalists whose values can be described as hateful and evil. If there was another American civil war, I wouldn't be surprised if a portion of the Americans refused to support their country because Bernie Sanders lost some kind of vote-contestant-off reality TV show.