If you have a garage that is available to park in, why would you not? Serious question, not being a smart ass...I assume everyone immediately thinks the worst after the last thread. Lol
Turning the wheels may not have helped if it was a complete loss of traction that initiated the slide. If the wheel has no grip, it's not going to change the vehicles direction. Perhaps water/snow flowed under the tire and froze over time, lifting the tread just enough from its expansion that the tire finally lost its battle with traction and just slid on down? Juat a random thought on that one, no clue how it could happen other than something like that or mechanical failure which I'm sure wasn't the case w 2 vehicles at the same time.
The parking brake is only as good as the grip the rear wheels have. Remove that and all bets are off. Did either of you by chance catch this on a security camera did you? Would be fun and nerve wracking to watch.
I once shoveled a retired lady's driveway like this when I was a kid for a 20. Learned my lesson that day I'd never own a house w a steep drive and live in a snowy area. Worst experience ever, I can only imagine how the expy felt as it slid uncontrollably down the hill.