Disillusionment through propaganda and well-timed leaks of stolen documents by a foreign, hostile power.
You could ask yourself why the Russians penetrated the RNC but decided not to release any dirt. Perhaps you could ask yourself if this has any bearing on Trump's Russia stance? Could he be unable to admit meddling for fear of undermining his very legitimacy? As a result of this is the head of state feuding with the unanimous conclusion of his (+allies) intelligence agencies good for anyone? Who would that be?
This is why the source matters. Because things are always leaked with intent, and by missing that intent and being distracted by shiny talking points you're missing the whole story. Both matter, equally.
Why do we have to "presume" they do about Trump and "know" they had some goods on Hillary?
If I remember correctly, it wasn't so much that many people switched parties, but that typical Democrat voter turnout was low, reflecting some amount of disillusionment.
Also, what you say about the Democrats' identity crisis seems to mirror what's been happening with Labour (the left party) in the UK.