Intresting experiment, Mike.
CO2 buildup being the trigger for your nasal breathing seems like a reasonable conclusion.
Both deadspace and WOB should be factors, as well as your excertion of course.
Next time you snorkel exhale through your nose to eliminate rebreathing.
Would be intresting to see how much of a difference that will make.
For some odd reason I've always done that when snorkeling, before I ever heard the term hypercapnia. Probably because bubbles seemed more like diving when I was a kid. Don't remember ever having a CO2 problem or hycap symptoms, though in the last 20 years I've only snorkeled once or twice.