If you read the articles, he suffered from a sleep disorder where he couldn't sleep for days and days. It looks like he was at the end of one of those periods, and he fell asleep in his car and died of heatstroke.
Apologies for the pl, but I have a similar disorder. It's hard to describe just how bone deep the exhaustion goes. Most people who have trouble sleeping can usually catch up at some point, but with a disorder like this there is no catching up. You go and go until you fall over, and the sleep you do get is closer to unconsciousness than it is to normal, and you can sleep for thirty hours straight and it's never, ever enough. I can well believe that this poor man couldn't fight the urge to finally sleep, and when he did, his service dog was unable to wake him and died too.