There are chords (chorda tendinea) that function as a "hinge" and attaches to the heart wall and valve leaflets. Prolapse happens when the cord is somewhat more elongated than normal. The leaflet under pressure bellows into the atrium and doesn't seal the valve resulting in back flow (leakage) of blood when the ventricle is contracting.
Sometimes the papillary muscle (area where the chordae is attached) may be damaged from an MI or virus, etc. causing the chordae to be elongated. If the papillary muscle returns to normal size or a reduction of inflammation, etc., the leaflets could possibly again close the orifice adequately. If the leaflet is inflammed or damaged , that will also cause leakage. If the inflammed or damaged leaflet heals, it can then possibly readjust to a better seal.