Alternate explanation: The clock stopped before his wife died, but the nurse recorded 9:21 as his wife's time of death, because she determined the time by checking the clock, not realizing it had already stopped.
Things like this always remind me to doubt clever-sounding explanations of phenomena I wouldn't actually have predicted in advance. Obviously, "not supernatural" is a very strong bet - but the specific hypotheses? Those are less obvious.
Here's the new thread for posting quotes, with the usual rules: