They were either super day-hikers, starting at Onion Valley or Horseshoe Meadows, and exiting over Trail Crest, or lost hikers.
Once in a while Whitney day hikers miss the "lower Trail Crest" junction of the JMT and the Whitney Main Trail, just below and west of Trail Crest. This usually occurs when they are descending from the Whitney summit. They make a wrong turn and descend down past Guitar Lake. At some point they realize they are heading deeper into the backcountry, and have to turn around and climb back up and over Trail Crest.
Unless you asked them, there is no way of knowing what their story was. Most of these marathon hikes (or huge mistakes) end up ok, and it is only a small number that wind up requiring any type of rescue.