To make it short and sweet, somehow, the tanks in one of my maps are magically shifting into tank mode without the ability to do so. I disabled "Tank Siege Mode" in the special abilities section, and yet they still manage to go into tank mode. The ability has been disabled for all players, so I think the fact that the units switch from an ally player to an enemy player has little significance.
its unseiging when a comp player controls it right? if so, thats cause computers always have the unseige/unburrow abilites, even when the seige/burrow are disabled.
The computer never unsieges (or sieges) the Edmund Duke (Siege Tank) hero, so use that.
or just use a neutral player and turn off ally status, that'll prevent the automated AI scripts.