The idea comes from a document that says Bush was disqualified from flight, in part for not showing up for a physical. These facts were contained in documents "released by Bush." It was wishful thinking that there was a direct written order that Bush ignored.
http://www.cis.net/~coldfeet/grounded.gif <-- See #6.
Was it in all honesty wishful thinking, or was somebody consciously misphrasing it - AKA lying?
If we can pin-point the original lie, there is a chance we're close to the source of the current forgery.