it's just another biased punitive mechanic meant to fk us in the butt... If you ask me it's intentional to make it so punishing and unforgiving, in a way they game's nullifying hours of effort leveling the comp, destroying an entire army in the process if they were leading a party, and forcing the complete loss of their gear + 1 RNGed companion nickname that you may or may not be able to recover in the future.
to it should be removed from the game unless they add meaningful dialogues and make companions actually immersive, otherwise just kill the code that does this and be done with it. In fact if a mod's made so that we gain more control over these deatails I'll def use it.
On a side-note, under a medieval setting, an action like that would be considered desertion and would be punished by death on the spot. Companions shouldn't have any say onto leadership actions (yet we must spend influence if they are leading parties in your army), while the most they should do was like in Warband where they complain and you get a speech check over the issue, if it fails enough times the comp leaves, but doesn't vanish.
Overall bannerlord's mechanics look rather harmless at first glance, but the details that added Warband's positive little depth are excrutiantingly bad in BL, this is just another one of those. There's also a second layer of stupidity over this which's that one of the factors intervening with this mechanic's how much relation you have with a companion, and the game provides nearly zero interactions that allow us to improve relations with both companions and clan members. It's very retarded. So far the only way to improve relations with them that I've found is to make them lead parties and repeatedly save their asses / help them defeat enemies. Not very feasible for too many reasons.