Help me about Dominions 2

[Targa]

If I had to guess I'd say that your archers were hitting your heavy cavalry and though they didn't get hit often enough to kill them, it demoralized them enough to make them rout. I also agree with Phil that heavy cavalry tends to rout fairly easily. In general, I find that any units with less than 12 morale are almost useless after the first few rounds of a battle. But I'm still a newb myself....