It's not from no exercise (although in a minority of cases that may be a contributing factor). There are specific reasons why shoveling snow is dangerous for those advanced in years:

http://www.advocatehealth.com/cmc/info/library/ham/win01/chmc-aa.html?heart

Many people underestimate just how strenuous shoveling snow can be. The increase in physical activity requires your heart to work harder. What’s more, cold temperatures can increase the viscosity, or thickness, of circulating blood. When combined with physical exertion, this increase in blood viscosity can raise the risks for clotting and heart attack. If you already have heart disease, shoveling can put you at even higher risk for cardiac complications.

and for those younger (it's good for them):
http://newsroom.msu.edu/site/indexer/1799/content.htm

If you’re a man of around 40, or a woman in the vicinity of 50 and you feel guilty about asking your son or daughter to shovel the snow off your driveway or sidewalk, don’t worry, it’s probably good for them.

When a man reaches the age of 40 or so, and a woman 50, blood coagulation potential starts to change,” he said. “Generally speaking, that’s about the time you might want to hire the son or daughter or grandchildren to do the snow shoveling.”

As people get older, he said, their blood has more of a tendency to coagulate, which can cause blood clots. At the same time, the blood loses some of the enzymes that can break down clots that do form





>And every years, the incidents of heart attacks soar in emergency rooms from men who do not do enough exercices... ;)
>
>>Every year when the snow falls, the incidents of heart attacks soar in emergency rooms from men shoveling snow...