Must be Gremlins!
Not sure how those are dealt with properly. The Shop Manual is notably silent here.
I would mention the typical plug wire terminals are springy just as you describe they can be opened up and widened a bit, and should just snap fit into the receptacle tightly with good solid electrical contact as they are inserted into place, see that each terminal are installed all the way home to the bottom of the cap receptacle. Then slide the boot over the whole shootin' match. It won't hurt anything if the plug terminals are shiny copper or brass either, a small piece of sandpaper makes quick work of this. It is important they fit tightly, not just to keep them from de-installing themselves, a lot of current flows through them and arcing and burning can result. Never had them pop out before!