Sounds to me like the conquest mechanic. Basically when you kill enemies of your faction as well friendly factions within their sphere of influence you create 'conquest' in that area. That area then begins to spawn NPCs of the faction that has gained conquest in that area.

You can see which faction has conquest and where on the table of command. This wiki should also cover the topic if you want to understand the mechanic better.

Yeah I know about Conquest. I guess it's probably that...

I might actually turn it off because I don't really like the way it works XD

Yeah fair enough haha. You can still check on the table of command. Who knows maybe there's something else at play here.

Gonna second this and say I've noticed this exact same issue. Gondor soldiers showing up in Rohan for some reason. Don't think it's the conquest mechanic though, because this was the first time anyone had been to Rohan.