Originally Posted by
Antiochos VII Sidetes
When it's one full stack versus another full stack, and especially when the AI is attacking -- things tend to work normally.
However, I urge you to play a campaign and attack an AI army with another AI army reinforcing it. The reinforcing army will sit on the edge of the map and not join in until you go after it and attack it.
I've had it happen three out of three times in battles of this sort.
Moreover, besiege a city until the AI attacks out of it -- their behavior will be strange, or at least in has been in the couple battles of this sort i've had.
If it's just a custom battle, or an army fighting another army 1-1, everything seems to be okay. It's only in the two specific instances I enumerated above, at least in my experience.