I think that's the only thing that makes sense even though I'm not positive heat related reasons totally make sense. Just hard to imagine 2 adults, a toddler & a dog all died at approx the same time from the same cause.
These people were world travelers who had hiked in all kinds of conditions. They had been living in the area. But they had moved from San Francisco where it's much cooler. And this year, well, every year, is hotter & drier than the prior.
We were hiking on a trail on the other side of the road from Hites Cove/Savage Lundy along the main Merced this past April. We live in MA but go out west frequently. We turned back one day in April. It was so hot, no shade, so dry-it was actually unpleasant despite hiking along the river. There were algae signs up although there wasn't any then.
We also visited Yosemite. We even went to Glacier Point Rd which was an unexpected thrill as it had never opened this early. Bad news is it was open in April instead of June because the snowpack was so little thus increasing the overall dryness which resulted in the current fires which made me evacuate my 23 yr old son, who we had just moved out there, from the Dixie Fire.
So my Occam's Razor is heat. I also only heard about one water bladder. That doesn't sound right though so don't want to assume that's accurate. Experienced hikers don't "share" water. Every adult needs to carry adequate, meaning worst case adequate, water for themselves. At least I follow that scrupulously out west-much more so than back in New England. And these days it is literally a life & death thing. You dehydrate so quickly out west right now.
The current moisture levels out west are the lowest ever. EVERYTHING is dry and flammable. The conditions are oppressive even for seasoned professionals. They had to cancel the sheep count at Anza Borreago this year after one person died and another was hospitalized due to heat.
BIGHORN SHEEP COUNTER DIES IN ANZA-BORREGO DESERT, ANOTHER HOSPITALIZED; FIREFIGHTER COLLAPSES DURING RESCUE
Heat is what makes the most sense to me.