What causes rebels to relocate overseas?

[Wanaro]

I'm doing a 3 mountains runs and I'm trying to understand what causes a religious rebel stack to relocate overseas. In my current game I am trying to get Coptic rebels from the Ethiopia region to jump to my provinces in SE asia. Frustratingly they have taken the 3 provinces I was using to spawn them in but they refuse to relocate to provinces in my core region. I am currently Hindu, as originally I decided it was the better choice, but now I want to switch. In previous runs I had the same issue trying to get Hindu rebels to jump from Champa/Khmer region to Manilia. Twice they would just sit on the provinces in SE asia and not relocate, and on the third attempt, just before I was about to give up and stab up to get the Animist option to convert to Hindu, they started to relocate. I have no idea why they move or don't move... can anyone shed any light on this? Thanks