I would describe it more as fragmentation. Look at how contentious the 2008 Obama/Clinton primary was, and how contentious the 2016 Sanders/Clinton primary was. There's no glue holding the Democrats together anymore, and all attempts to re-establish a hierarchy have failed. This sort of Hobbesian all-for-themselves infighting really brings out the crazies. Crazies don't normally have a shot at getting power. When the Democrats were on their A-game, like in the Bill Clinton days, crazies were kept out of sight. But now the crazies smell blood in the water and think that now could be their time to shine. But it's just a symptom of the entire party fraying and coming apart.
The Republicans were having this problem to a lesser degree, but they have learned fairly quickly that they must submit themselves to the golden-plated whims of Emperor Trump.