So ... why did the Vikings settle down?

There is no simple answer to this, just as there is no simple answer as to why they went on a rampage across most of the Medieval Western World and points east and west in the first place.

The single biggest factors where: they were Christianized, they were tamed by their own growing states, they were increasingly running into stronger states with stronger armies.

Whatever demographic force had built up in Scandinavia was spent, not to return until the second half of the 19th century when they all decided to move to Minnesota (where second generation Scandinavians not only won't eat luttefisk, they won't even stay in the house while it is being prepared).

Local governments in Germany, France and England were now able to take on Viking armies. Meanwhile, back home in Scandinavia increasingly powerful ruling elites not only did not want to go a-viking themselves, they didn't want any of their people taking off either.

But the biggest factor was Christianization. Arnold Toynbee once imagined what a possible pagan Scandinavian civilization might look like and was impressed. He thought it would have been as remarkably creative as ancient Greek civilization. He lamented, I think, the fact that this civilization was killed in the cradle when its members turned to Christianity. Once that happened, the folks who had produced the Vikings became peacefully (usually) integrated into the rest of the Christian commonwealth.

Still, Toynbee might have had a point.