Alas, the problem is that I have no idea HOW it happens - if I did, I would stop doing it...
One of those rare - but, apparently, Not Impossible - occurrences. My only guess is an error in key combination during the cut & paste operation: Wrong Combo; Random Extra Key hit - the old "Phases Of The Moon" answer (!). Am not able to Deliberately replicate it - but have not been willing to spend any significant amount of time to do, given that this (again) IS a "rare" occurrences.
Somewhat Related Aside: Do recall having frustration with an ANDROID app - with a Notes module - wherein the ENTIRE CONTENTS of a Note were "pre-selected" WHENEVER the Note was opened. Therefore ANY CHANGE to the Note would wipe the entire contents UNLESS you carefully UNselected the contents! I think it eventually got "fixed" - - - either that or I moved to different Notes app!