My guess is that the axle was slightly out of the dropout when I tried to close the lever, causing me to loosen the quick release, but then popped back into the dropouts when I mounted the bike, leaving the wheel pretty well lined up, but the skewer loose. With the heavy load on my back and on the bike, I might not have noticed a slight brake rubbing or a bit of a wobble, and gravity kept the wheel in place until I lifted the bike.
I'll look at my dropouts later and see if there's a bit of a ridge ("lawyer lips") around the slot...that would fit my theory.
R