“we saw the hands go around and around really fast for 15 minutes and then reset itself at the exact right time.”
Normal behavior. The “atomic” clocks for home use are actually radio controlled clocks...they get signals from NIST via very low frequency radio. Most of them re-calibrate once a day. I have 2 analog versions that do more or less what you described at about 1:30 am local time every day.
It has never done that again in 7 years.
The hands both moved continuously as fast as they could for fifteen minutes without stopping. The clock only did this twice. Once the day she died the other when the women from the funeral parlor came over to discuss the plans for the funeral.