I think the Flynn thing probably is in part due to this. Trump likes his Fox "News", Hannity, and Nazi sympathizers (Gorka). I doubt it's a coincidence that all of this pardoning Flynn talk is coming out simultaneously from Fox, and Trump.
As far as the "Witch Hunt!", tweet, it's Thursday. He's upset about the Cohen situation, realizing he may end up in a cell next to him. Or he knows Mueller's going to drop something else tomorrow, given it's Friday, and Mueller loves his surprise Friday indictments.