It had to do with inheritance. Younger sons in the viking era did not inherit the farmstead and had to seek their fortunes elsewhere.

Anytime they had a population explosion, younger sons would have to go A Viking.

This is often said (also about the Viking age) and it seems such an incomplete explanation to me. Normally if more people are born than the land can feed, then vulnerable people (eg babies and kids and elderly) die at higher rates until the balance is restored. Why would Scandinavia be different?