What is happening here?

[Ivan Rogov]

I have 20,000 troops with 100% morale under the command of a lvl 24 Marshal attacking 2100 Cuman troops under a Duke (Chief?) with a Marshal score of 9 in a Plains province and not across any river. My troops collapse within thirty seconds! WTF?!?!

 

[Veldmaarschalk]

Have you save and reloaded with your troops mobilized, shortly before the battle ?

 

[Ivan Rogov]

Have you save and reloaded with your troops mobilized, shortly before the battle ?

No

 

[Jolt]

Then the only plausible explanation is that all of your army's levies except that of your marshal were composed of Anonymous Commanders.