Partly it’s a misunderstanding of his democratic socialist politics. Partly it’s an understanding but a personal disagreement with them. It’s lead to several smear pieces about him on their site, some of which come across as downright conspiracist, predicting he will do his darndest to defeat Hillary even if that means helping Trump.
It seems the latter have just been proven wrong. It’s a cautionary tale about not letting your personal emotions about something get in the way of being able to perceive and interpret facts.