Near the end of The Last Jedi, after
astral projecting himself to the battlefield and allowing the rebels to escape, Luke just disappears, causing his body to disintegrate.
As far as I know, this isn't a side-effect of "too much force usage", so what is the actual cause of
his conversion (presumably) to the Force?
I understand from a purely plot perspective, it could be argued that
he intentionally converts in order to provide more "power" to Ray (for lack of a better word), as the Force keeps balance and would provide either Kylo Ren with enough power to match both Luke and Ray, or would send something over to the Dark side as an equalizer. This explanation however does not seem to show us why he disintegrates unless by choice, which given the timing seems like a bit of an odd choice to make as we don't have hard evidence that Ray has been fully trained in not only fighting with a lightsaber, but also in basics of how the force works.
I just want to know how it worked in the movie, so please only canonic sources.
Note: The linked duplicate asks how he astral projects himself, which I am not questioning in this. I'm asking closer to
what was the cause of his disappearance, seeing as we've never seen a "running out of Force power" as an actual side-effect of using the force.