But do you have any idea how or why I was able to claim the Byzantine Empire myself? It's quite a significant change in the game, and I would like to know how that was made possible. Also so that I might be able to do the same trick again against Francia or England, for example.
My best guess is that it had something to do with family. I was able to marry the Byzantine emperor with one of my daughters, but that was a few generations ago already.