Bug? Poor AI behavior

I was playing the Campaign, and had to battle a Drake, and two other mobs.  All my Swordsmen quickly died, and then I noticed a funny thing.  The Drake refused to attack my archers. ( The archers you get from on of the other civs. )  My two heroes were running around and around the archers, being chased by the Drake.  The archers were mostly missing, but averaging about 1 hp damage every other turn. On turn 145, they took the drake down.  It refused to attack them.  Something is wrong with this picture.