Today I saw what you were talking about. This behavior seems to be idiotic, but it is not such idiotic as it looks. The purpose is to make choshas waste a "move" - summoning that wormlike creatures or making them poisonous. Choshas just defense instead.
Something really stupid and not so funny is when AI attacks the red dragons when they are able to retaliate. Of course, the weakest AI unit attacks first, but it seems they forgot about the friendly unit behind of them.