Not sure what the extent this is, but whenever spend all my money maximizing crusaders levels it unlocks both crusaders from that slot, from what I know, I can't get any of the abilities for them though so it's kinda useless.
https://gyazo.com/4b5694852d55516899ada67ed65584cb

It happens every time when I have neither of the crusaders in the swap slot purchased, have two slots open in the formation and hit "max the levels" button. It becomes much more obvious now that we have swappables for the early crusaders.