Keep in mind that official weather records are supposed to be taken in the shade.
I live in a climate similar to Pleasanton's, and while the official temp from the weather station down the street is 72F, it's 94F in the sun on our back patio. The weather station is at the local school, two blocks away. And, indoors, on the shady side of our house (we planted shade trees as we have no A/C) it's 72F.
So, in looking at the trails that run along the top of the ridge, there's very little shade. So while it was 95 in the shade at 11:45 according to LocalConditions (which uses multiple points of shade measurements), it was hotter in the sun.
(BTW, those record-breaking temps at DV, which just claimed the life of a hiker, are taken in the shade, 8 feet above the ground - as the ground also radiates heat).
Ask Tom: Are official temperatures taken in shade or sunlight?
And...as the human head needs to be cooled off more than the rest of the human body, and because it has the sun shining directly onto it, the brain can heat up (like any other hard object subjected to the sun). I'm not saying that's what happened here, but something happened here, and misadventure/medical issue is what comes to my mind first. A lot of people think that water alone is enough (helps if it's on your head, really - I don't know this man's hairstyle, but longer hair and curlier hair retains sweat/moisture longer and therefore provides some counter-action to the sun).
BTW, that's why humans retained hair on their head, according to biological anthropology. As evaporative cooling in our ancient homelands. When I tell my bald-by-choice students that they are more likely to have heat exhaustion/stroke than, say, myself or my colleague with the very curly hair...they don't like to hear it. But it's true and people who work in the fields in California know to wear a light colored hat and those who wear dampened straw or cotton fare much better.
Was Phil wearing a hat designed to reduce heat on the head?
I just reviewed a few articles on heat stroke, and it turns out that from age 35 onward, one is more at risk, and at 75, one is way more at risk (of course). Risk of heat stroke in the same conditions, if 34 and under was significantly lower than in the over 35 group, with each year adding some risk.
From one article:
//In healthy individuals, even relatively small decrements in hydration status (1% of body weight deficit) can impair the cardiovascular and thermoregulatory responses.//
That means a youngish, conditioned person can still be more at risk due to the increased sweating that direct sun produces. That's less than a liter of water.