Thanks for answering my post.
Apparently, the only way ethanol could oxidise to Acetaldehyde is through alcohol dehydrogenase, which is handled by enzymes;
in our liver and in yeast for instance.
I cannot think of another chemical reaction happening in a bottle of wine which could induce ethanol oxidation to acetaldehyde.
So, the explanation for ethanol oxidation (in wine oxidation):
some yeast is still alive and has metabolized ethanol into acetaldehyde.
Or in presence of some residual sugar, some yeast has metabolized sugar into ethanol; in presence of O2 around, that might have gone wrong (O2 binding with a degraded ethanol) and ethanol turned into acetaldehyde or another nasty thing.
That kind of mess could probably happen in a fermentation too;
in bad winemaking practise: like exposure to oxygen during fermentation (when it's not done at purpose).
In a nutshell, that's got to be at enzyme level.
Therefore, not a "natural" chemical reaction.
Does that make sense ?