I know that its extremely speculative and very farfetched but I was reading some time ago Sagan's The Dragons of Eden where he was speculating the "fear of falling" as an innate, evolved mechanism rooted in the history of human ancestors. He cites Washburn's theory that our ancestors lived in trees, where falling was a frequent and deadly danger thus the instinctual fear of falling is specifically tied to the dangers posed by gravity to tree-dwellers. In this prism we can consider the need to lay down/flat or secure our position during PEM or even at baseline ME/CFS as an internal danger response to whatever goes continuously wrong in cellular level which can cause the aforementioned incidents i.e "falling from the tree".