Actually it is not really a mystery at all. There has been a thirty point or more swing in approval of Medicare for All in just a few months. That type of stuff really does not happen. How do you explain it. Well the journalists throw up their hands and say, these things happen. Hmmmm.
Let me give you an answer I am pretty certain of. Polling outfits are most probably changing the tone of the question, adding stuff. Any fourth year psychology student can tell you that if you ask a question based on what you would gain, people will be all for it. If you ask a question based on what you will lose people will be against it. It is just human nature.
So first, the questions must be exactly the same every time they are asked for the findings are meaningless.
The big finding from Gallup for instance is that people are really, really behind universal health (71%) (give them something) but that goes way down if you ask them if it means giving up their doctor (take away something).
A far better more stable poll is the Kaiser Family Foundation poll which is at 56% (pretty damn high for the US) and going up incrementally for the past two decades.
What is important here is that the media is trying to control the narrative. Think back to the debates. Medicare for All questions were all asked from what people are giving up. So here is some advice to candidates. Do not allow the media to set the narrative. When somebody like Chuck Todd asks, innocently, innocently I tell you, “Are you worried that people might think they are giving up to much” DO NOT TRY TO ARGUE THAT THEY ARE NOT GIVING UP OR THEY ARE GETTING REPLACEMENTS. You must flip the narrative immediately. Let me tell you what people will be getting from Medicare for All. And if they must, let me tell you what they are losing from the current medical system. Over and over again, never respond to a question that is asked within a deficit framework Ever!!!!