Not my chosen explanation, but it seems to have possibility. A localised tremor, perhaps in the immediate vicinity only. Having searched the forum using the freetext search I got seven hits. So why not. Or how?
Minor earthquake dislodges some sheet ice above them, this bumps it's way to the tent. The event is over. However panic sets in and they exodus to the trees in whatever state they were in. Arrived at cedar tree, make a fire and climb cedar to see if hill has collapsed. Yes it was dark. Thus follows bad decisions and unlucky slips and trips.
Open to suggestions. Requires no leaps of faith. Main question - how far down the hill would you expect experienced hikers to go before they realized they were in the clear? Turn around or head for cover?
Main drive of this is the headline photo of the freshly discovered tent with chunks of ice or snow on top and at the entrance. Let's assume the site wasn't doctored.