I know all the logic behind General Relativity quite well. However, what seems to be lacking is the model for causation of time dilation. The generic notion is that if an observer accelerates (change of inertia) his aging slows down relative to anyone he left behind. So now after acceleration (and in constant relative motion) the observer is in reality supposed to be aging slower relative to anyone left behind.

So the question is:

Is there any model for the causation of time dilation or is the causation undetermined?

Essentially, how does acceleration affect the relative time dilation between any two observers?