It’s all in the open, you can go dig around for reasons. As usual there wasn’t a single simple one. Neither was it some kind of complete fallout, we e.g. collaborated on translations and I have been in contact around various things with the one that forked.
I am not the creator, funnily that is/was one of the Lemmy creators: Nutomic :)
I am a syncthing co-maintainer that kept the android app on life support since a while.