On occasion, I'll open a bottle of wine from a few years ago and it will be
fizzy. My question is this: Is this more likely the result of incomplete
alcohol fermentation or is it likely due to an MLF that occured in the bottle?

All of my wines are sulfited to the appropriate level. Often, if I just let
the wines breathe a bit after opening, the fizziness disappears completely, so
it hasn't enormously affected the taste of the wines. This occurs in both reds
and whites.

Asked another way, if you have an incomplete or stuck MLF, and then sulfite and
bottle, what's the chance that MLF will continue.

Lee