Why some people drop dead while shoveling snow

In French Canadia, No One Can Hear You Scratching Paintwork!

Each approaching blizzard brings warnings about the dangers of shoveling snow, an activity that sends thousands of Americans to emergency rooms each winter. Don't tackle the walkway if you aren't in good enough shape, say the experts. You can get hurt or, even worse, trigger a heart attack or stroke. Medically speaking, a person is indeed more likely to keel over while heaving snow than, say, jogging on a treadmill. But why?
(independent)