If he was at 10,000 elevation he could have had a problem related to lack of oxygen, compounded by walking any distance. An unknown circulatory or heart condition may not show up until you reach a certain elevation then it gets you. It can also cause a person to make unwise decisions without them realizing it. I've a worn out heart and it happens to me.
 Sad though whatever caused this to happen. Condolences to the family