All I can say is the game does not turn on computer control all by itself
BUT
IF you ran a turn under computer control for both sides the game can retain the computer control mode for what would be your human control side
ALSO......... if you have not run your side under computer control at any point and ONLY played out whatever type of battle you are playing under full Human control there is NO WAY I know of that the game will change a couple of units to Computer control that started out set up as Human Control
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"You are never to old to rock and roll if you are too young to die".--- What do you expect to be doing when you are 80?
Have you opened the unit menu at all during this battle? If you press the H key while that menu is open it is possible to inadvertently switch units to computer control. It will switch the unit at the top of the list the first time you press H while on that screen and will go down the list from there with subsequent presses. If the formations that were under computer control were on the very top of the unit list that would be my guess.