If they did indeed take the HC OHV Trail/HC Road — MOO, perhaps on the way down to the river, they encountered an animal that was threatening...? Or had an altercation with a single angry person or group of folks who were intimidating...? Or, as someone has already mentioned, they were witness to something...? This would lead them to consider another route back out of the canyon — MOO.
MOO, in these times we are living in (global pandemic, job recession, polarization, a notion of "outsiders", housing crisis, the disparity between the have and have-nots, etc.) it is not hard to imagine encountering agro people, be it towards (one's) happiness, status, race, etc. even on a hiking trail

— MOO.
Another thought or theory — since it is suspected that they started out down the HC OHV Trail/HC Road — MOO, perhaps they turned around (for one reason or another) and decided to explore the SL Trail. They got halfway down the SL (for some amount of distance), but turned around. It was too hot. And baby and pup were suffering symptoms of heat exhaustion. Perhaps the baby succumbed to HS, and they did not realize it right away, and they used most of their water to cool off the pup —> MOO, parents are then traumatized, distraught, feeling guilty, overheated, fatigued, and suffering from poor judgment and lack of water and unable to press on together —> since EC was found higher up the trail, perhaps she was going for help. She maybe could not carry the pup, so she secured him to JG who stays with the baby, and heads toward the truck. MOO, she is in shock, overheating, grieving, and extremely exhausted. As mentioned earlier in a WS thread, she was recovering from a TBI.
< modsnip - quoted post was not linked>
MOO, perhaps she had a seizure or another type of TBI side effect (problem with cognition, senses, etc.). I have a close friend who has been recovering from a TBI from four+ years ago. The long-term effects can persist for years or throughout a person's life.