As far as I've observed the AI always keeps its troops on its capital if it can. If you cut off their capital from their troops in a war they will repeatedly ask for military access as they want to put their troops on their capital. Scottish capital is Lothian, which borders England, so they sit their troops there and do the normal AI low maintenance while not at a war/preparing for war.
They'll even put all their troops on an island if its their capital (looking at you every Caribbean colonial nation). I'm not sure what the solution is but always keeping troops on their capital is almost always a bad idea since it's so predictable.