Beginner question : religous conversion

[Magean]

Hello, I just bought CK2, during the last price drop.
So, I started a game as the Provence duchy and I'm assimilating game mechanisms.

But I saw something weird. The Fatimid Califate rules over Apulia. I thought they conquered the duchy, but in fact Apulia was still ruled by Norman lords. However, these Lords had switched religion to Shia Islam. So I'm wondering how this could have happened : after a war, the former overlords should have been replaced with Egyptian new ones. Perhaps the Lord of Apulia has voluntarily come to heel to the Fatimid calif, however after checking in the decision folder, it looks like it's only possible to convert to your liege religion when it's in the same group as yours.

So I'm trying to understand : how ? What happened ?

Thanks in advance

 

[suchiuomizu]

One of the options a liege has is 'demand religious conversion'. They have to like their liege enough to go through with it, but it can happen.

 

[Darkgamma]

+30 is the lower limit, I believe.

 

[unmerged(397168)]

The muslim invasion casus belli doesn't overthrow the old rulers, it subjugates them. The old rulers get a relations bonus with their conquerors for a short while, which is often enough to get them to convert if their new liege asks them to. This causes the shi'ite Normans.