The only explanation I can think of is that they bought the "he can't be bought" line. However, I outlined in another post why this dude has the personality of someone who can be bought more so than any other person. He is about the "deals" and is an egoist. He will definitely use the opportunity of how he can make some great deals for himself.
This guy can't even articulate basic policy, and people are supposed to believe he is in this with hopes of actually being administrative instead of just out for self 100%? Not naive to think politicians aren't self serving but I honestly believe this dude probably tops your average politician in that regard.
Anyways, its not a genuine ideological shift I think. I think its just sour grapes for people who make the incredible leap from Sanders to Trump when Hillary and Sanders agree on more things than Sanders and Trump.
Its so ridiculous that I personally have stopped caring either way.