"a traitor to our people" - under which circumstances does it happen?

[Arcimaestro Antares]

and how can I modify it?

Sometimes, huge armies betray me, during a siege or simply while moving to a settlement. Even armies with a general. I noticed this happen also to AI factions.
I would like this didn't happen to huge armies and to armies with a general. How can I modify that?

[lolIsuck]

I guess generals with low loyalty and faction leaders with low autority.

[Moon]

Open the descr_strat and scroll down to this part, add a semicolon ";" and save it, that should stop your generals from going rebel on you.

marian_reforms_disabled
;rebelling_characters_active
gladiator_uprising_disabled
night_battles_enabled
show_date_as_turns
brigand_spawn_value 20
pirate_spawn_value 28

[Arcimaestro Antares]

thank you.
I suppose then there isn't a way to avoid that huge armies betray me...
Only small armies maybe left without orders for a few turns, should betray. Not during a siege that they will likely win.

[Squid]

Realistically, its probably more historically accurate that larger armies would revolt than smaller ones, since with a larger army the general would have a better chance of accomplishing something.