Psychological warfare.

[Tora]

Had a Muslim rebellion in Russian Crimea consisting of 16 ballistae, a previously observed phenomenon, I gather. It was reinforced by about 120 men and a 2 star general from Khazar. I left about 120 and a 4 star general thinking- Ah well, the ballistae won't even come into the attack.
Wrong.
The body count was fractionally in my favour when the white flags started waving and my army legged it.
It would seem that the number of ballista crew, although on the other side of the map, well out of range and hardly visible, had convinced the computer that I was outnumbered about 4 to 1 and that the fear factor as well would be enough to persuade a 4 star, 14 valour general that it was time to make a hasty exit.

A completely unlikely uprising and a completely unlikely outcome.

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"The last time I saw your father, he'd just charged Constantinople when they shut the gates on him.....
Ten thousand of the Turks were there armed with scimitars, and your father with a small knife for peeling fruit."

Blackadder 1.

[Spino]

The unusual composition of the rebel force is reported to be fixed in the upcoming patch.

As to the routing of your troops you either caught a bad break (helpless in melee or not, artillery units do inflict fear) or you failed to take into account your general's Vices and Virtues. Four stars and high valor is good but if he's a serious closet coward all ( up to -9 morale modifier!) then it's no surprise that his men broke.

[Satyr]

When lots of artillery are present I never let the comp decide the battle. It is so easy to run cav over all those while keeping your troops out of the way that I always fight those myself. Same with lots of archers and peasants, even though I may be vastly out numbered, I will command the fight because they are so easy to rout with cav and the comp won't take that into account.