I think UX Design is subject to non-disruptive simplification. Interactions are made simpler because of asynchronous technological progress and more educated users.
The come back of JS, the rise of: new technologies, action verbs (Like, Share, etc), Meta Data and voice recognition are all examples of this slow process (non disruptive simplification): it took some time for each to be mature (usable) and to spread to the web ecosystem.
Another example is form inputs and controls. Some very smart input fields exist on some major apps and take a long time to be replicated onto others apps. Often because the front-end logic is extremely back-end demanding.
To answer the questions in your last paragraph, I think it is a learnt behaviour which with time become more and more natural and then expected.
UX is in the same boat as most of the IT related specialties, it can be done right the first time (satisfy users' expectations) and it needs to be revised to reflect evolving users interactions.