Bill Tindall, E.Tn.
I failed to tighten the nut after the last change.
Happens easier on a grinder that has a wheel with more mass. I have had this event happen many times on a grinder where I didn't snug the nut sufficient. When the grinder spins up, inertia of the wheel resists rotational acceleration. Friction of wheel to flange to nut tightens nut. Grinding will appear normal. When machine is shut off the wheel decelerates and the process described above reverses. If nut is loose enough it will loose friction and unwind.
Thread's "handidness" is designed to tighten spinning things upon acceleration for a loose nut in this situation would be disaster-grinder, saw blade, lawn mower.
When installing a saw blade I pinch it between fingers and snug nut. Enough for my saw situation. If you need more, jam a push stick against the blade.