What Causes Time Dilation in the Twin Paradox?

[Dale]

This seems to be the one factor common to all approaches for explaining the twin paradox. If you use a consistent inertial frame it works. If you don't, you need to do something extra ...
I don't think we are getting any further than that, and I don't really think there is any further to go than that.

I agree 100% with that. And really, that is the whole point of the relativity postulate: if you use any inertial frame everything works the same. (it even rhymes )