Apart from certain units (elephants, chariots, catas and infantry who like it au naturel etc), being flanked and being hit with cav charges, what else brings down morale/causes units to rout?
Definetly getting shot at the back by slings or archers.

Once routed a whole mob of roman legions cuz i had 3 slingers at the back pelting them. definetly saw a morale decrease when the stones hit. Shaken to wavering. (did this in RSII, but eb using same engine)