I've been having a hard time finding historical data for that specific area for that day, but looking at surrounding areas on that day, I really don't think it was anywhere near 107-109 that day... even at the hottest point on that day, which would've been late in the afternoon. I also find it hard to believe experienced hikers with a baby and dog would have left in the afternoon. I don't care what that SF article reported. Reporters are known to stretch the truth or re-word things for dramatic effect.
If they had left that late in the day at that high of a temperature, that would have been uncomfortable carrying a baby pack from the start. I can't see someone continuing on a hike before they even left the parking lot. I also think it would be fairly obvious to LE it was heat related if all those factors existed. They're baffled, it doesn't make sense to them.
I live in an area where toxic algae is a problem in many bodies of water, usually when drought is at its worst in what wouldn't ordinarily be stagnant water. Typically it's dogs that suffer the most, but I've known a few people who have swam in an affected lake and had some minor issues, such as mild blistering around the mouth (they did not intentionally ingest any water, just swam). No smart hiker would ever drink straight from a river like that. They would have had to be in serious trouble to begin with to even consider drinking it. I could believe swimming in it, if you were not aware, but wasn't it marked? I would think most dog owners would know how toxic it is to animals especially.
All of these things could have happened, but they're not likely... not with seemingly very intelligent and experienced hikers. Could they have eaten something toxic on their hike? Possibly, but again not likely. I don't know these people, so there's a possibility they're a bit of risk takers, but I don't see it. If you're a forager, you know you don't eat random mushrooms on a trail.
I'd believe they possibly ended up in a little bit of trouble and things spiraled to the point of heat stroke and then death, but I think the real question is what happened to put them in that position in the first place. I just don't believe the conditions existed for that to have happened to them all in a short period of time. What surprises me most is that this trail has been described as "popular," so how is that no one came across them in that entire Sunday afternoon and all day Monday?
I think that the trail is popular in mild weather.