I am unsure, but I suspect they have a stopgap for war in bannerlord. A system that just randomly makes 2 kingdoms at war? As I've even had my OWN faction of just myself declare war on Sturgia while I was in Aserai lands.
I can't imagine why else it would say my kingdom declared war on theirs, especially since I can't even declare war on anyone as kingdom management isn't in (or wasn't at the time.)
Might explain kingdoms suicidally entering 2 more wars after losing 1 war badly... and not even finishing it yet.
It's the only thing that would explain that! I've had the southern empire, which was losing to the Northern Kingdom, declare war on the Western empire... then Sturgia... It did not go well for the southern empire. It seems completely random and might be.
Not much can explain entering a 3 way war when you are losing a 1v1.