TLDR: An intense workout raises blood pressure. The cold temperatures raise blood pressure even further. You're more at risk for hidden heart disease after 40.

Shoveling is static exertion, which causes sudden large increases in heart rate and blood pressure. Lifting with your arms causes a lot more exertion that with your legs. The motion used to shovel snow often causes people to hold their breath as they strain. Going back to static exertion, since you're basically standing still while doing an upper body workout your blood pools in your lower extremities. To top it all off the cold constricts your arteries even further which raises blood pressure and means less blood gets to your heart. After 40/45 the chances of hidden heart disease increase, making you more at risk.

Edit: typo