Unit Chosen From Carousel Replacing Unit Out On Field

Sometimes after the carousel, my collected champ will go to the field while one of my units that was already out on the field will be benched. More often than not, it'll wind up benching my stacked carry. This doesn't happen every time, but when it happens I don't always realize fast enough to fix it and it will cause me to lose rounds and sometimes LP. I can't fathom why this would be intended, but if it is, please explain why it is happening.