The reason may be this:
There are many pins/ribbons/medals worn by
military members that have the detachable clamps on the backs. You push the pin through, then attach the backing, almost like an earring backing. Those little backing things are called "
frogs," possibly for their squat, round shape. The
kenzan are somewhat similar(in
function, if not in appearance) possibly leading to calling them the same thing.
It is just conjecture, but if anyone has a better idea...