Help me about Dominions 2

[General Tacticus]

As a general rule, don't back up heavy cavalry with crossbowmen - you risk heavy losses from friendly fire...

As for what happened, what would make the most sense is that your leader(s) died (or routed), and all your army routed.
It is, I suppose, barely possible that some extremely unlucky dice rolls caused your cavalry to rout. What cavalry exactly are we speaking of ? Did they use spear, or did they use swords ? In the latter case, the light infantry, on an incredibly lucky day, might use their longer spears to successfully repel the cavalry, causing moral loss and eventually rout...