Troops wish to join you?

[Metathron]

Waiting for the Armored Princess, I recently decided to start up a new game as knight on hard. I was surprised to see a group of priests wanting to join me, as this has never happened to me before. I am not talking about the diplomacy skill because a) I do not possess it and b) that automatically gives you a small portion of the enemy's troops, provided they are the same as one of your stacks.

So, has anyone experienced this? Any idea what it's about?