In the best documented runaway escalator case I'm aware of -- see https://en.wikipedia.org/wiki/2018_Rome_escalator_accident -- it was determined to be the result of criminally negligent maintenance practices that included the use of zip ties to keep an emergency brake mechanism from engaging.

There was a similar incident this summer at a subway station in Atlanta but there doesn't seem to be a clear root cause published yet. As in Rome, the trigger was too many people on the escalator, but the mechanism is supposed to cope with overload by stopping the escalator.