Those are great pictures! Thanks for posting them.
Since we don't see the end of the hose, it's hard to tell what happened. Since there isn't any "special equipment" evident, I'm not sure he meant to check out. Although if he had something as simple as a plastic bag rubber-banded to the end of the hose, that would speak to intent.
If he passed out for any reason, (drank too much, 'ludes, medical event, etc.,) then simply lying on his air hose might have been enough to suffocate him.
I hadn't thought of any of that, badtom. Now we must wonder if he might've just been playing dress-up with an unattached gas mask...and then something went wrong. Shit does happen.