IMO when kydex is formed to a knife, it holds the knife in the seated position. When you draw the knife out, the kydex will spread alittle and spring back into place. after inserting and removing the knife, the kydex is forced to be in an un natural position (since naturally it sits in the closed seated position). That action will loosen kydex. (if you sheath and un sheath the knife once or twice a day, it will probably never loosen. You have be using it a lot, or just playing with it a lot
)
You can take a piece of kydex that is bent into the folded over position and completely closed. Then spread it a little ( alot of times) you will see it will not stay closed, therefor it has loosened or spread. Kydex has a memory of a sort and remembers its last position, thats how it works. When its last position was a little spread open (while drawing), and now the knife is re inserted ( it's in it's closed position). The sheath starts to remember a happy medium between open and closed. That is when you will feel it loosen.
Also bending the kydex back and forth creates heat. It has to, because you are forcing the electrons to move when bending back and forth, thus creating heat. It's micrscopic but I am sure the kydex knows about it
Also aggressive handles will wear a path making the sheath even looser, and feeling even more broke in.
But there is a point, where the kydex will not "loosen" or "break in" any more. It's a fine line, but it's there.
Al