Malolactic Fermentation (MLF) is a secondary, bacterial (not yeast-driven) fermentation common in cider and wine, converting sharp malic acid into softer, rounder lactic acid — a genuine acid-character transformation rather than anything to do with alcohol content or fermentable sugar the way primary fermentation is.
Not every cider undergoes it, and that's fine
MLF can happen spontaneously, from naturally present bacteria, or be deliberately encouraged with a cultured malolactic bacteria addition — plenty of excellent traditional ciders skip it entirely and keep their sharper malic-acid character intact, which some cider styles and drinkers genuinely prefer over MLF's softer result. It's a real stylistic choice, not a required finishing step every cider needs.
Why it sometimes shows up as a surprise, weeks after "finished"
Because MLF is bacterial rather than yeast-driven, it can start well after primary yeast fermentation has genuinely finished and even after a cider seemed stable — an unexpected slight fizz or softening flavor shift appearing in a bottle weeks later is sometimes this process happening spontaneously rather than a flaw or a stability failure, and recognizing the difference matters before assuming something went wrong or that a batch has spoiled and needs to be discarded rather than simply tasted and reassessed.