Hey Carl,
when power is cut off from stepper motors they loose their Electromagnetic Detent so they can hold their position only by the remaining Magnetic moment, which is usually not enough to prevent massive moving parts in the kinetic chain from moving them by gravity.
The remedy, however, is to retrofit an electromagnetic brake. It lets the stepper axis move freely when there is power, and when the power fails it clamps the stepper motor axis and prevents it from turning.
Welcome to the forum!