The Tur quotes his brother, R. Yechiel [and is codified le-halachah in the Shulchan Aruch] that if a bough falls into an oven on shabbat, you can't use that for fuel on YT the next day [I think; I'm not sure if I understood the case correctly] on the grounds that it's preparing from shabbat to YT.
But who was preparing? It was an accident - it just fell in by itself, didn't it?

But it cant be used on shabbos itself, therefore it is like shabbos is preparing it for the next day (which is why if it where yom tom before shabbos some are of the opinion that it would only be rabbinically prohibited).

but still, nobody is actually preparing anything so there was no preparation and therefore no violation of any prohibition. rather, it just fell on its own. Or is it a nolad issue rather than a hachanah issue: if an apple fell from a tree on shabbat = erev YT, what would be the status of the apple on YT?

The apple would be prohibited because of a hachanah, it appears that "hachanah" does not necessarily mean that the person cannot prepare, rather the day itself cannot prepare, and remember hachanah is a biblical prohibition.