Actually i have seen this happen quite a few times... whenever i have archers with stakes, i put the stakes in front of the infantry and after battle starts move the archers to the front to get a direct line of fire (which i feel is more effective). If the AI does not have infantry i will pull them back into the stakes to get the AI to charge into the stakes... But if dont time pull back properly, the AI cavalry will stop its charge and come around to my flanks. btw i play vanilla 1.02.