Why would a health 33 man pass out?

Could be a million reasons...

Heat, lack of food, etc. More info would be helpful.

And of course he may not be as healthy as he thinks he is. Only a doctor and a complete exam will tell you that.

Could be some underlying medical condition that he does not even realize he has. Could be heat stroke or exhaustion. Could be, like J_9 and Fr. Chuck have stated, other things but he would only know why when he gets a complete physical with blood work done.