I am beginning to believe the dog was the trigger. It has been established that the crew did make it to the South Fork, via Hites Cove. Bit of a bushwhack up canyon to Savage Lundy, completing the loop by climbing up the switchbacks (a bearish ascent) to complete the loop.
At any rate, the river canyon—the destination, shaded with cool water running through it, serving as an ideal picnic/rest stop—would mitigate any early heat-related issues, after five relatively easy miles over 3-4 hours (with lunch break).
The river, at near historically low levels, may have lured an excitable, panting animal to its deeper and more stagnant pools, a breeding ground for toxic algae, not necessarily fatal but causing unpleasantness.
.