They had 'issues' at home.
They opened up iron mines, and from that iron they produced farm tools. You can open up a great deal more land using a metal shod plough than you ever will using a wooden foot plough.
So, within three generations they had more arable land= more food=more children surviving to adulthood and healthier older adults too= population explosion and land hungry young men.
They traded, they raided, they traded some more, they settled, and it's noticeable that they were settled on what is now submarginal land in areas like the Lake District, not the best land, the stuff that the locals weren't likely to dispute much (I walked and dug sites in the area, the Viking farmsteadings were all highish and poor land)
In time they became integrated.
In Scotland's western isles they married into the local hegemony and it created what the Scots call Galgael. The foreign Gael.
The first two generations we have pagan graves (grave goods, etc.,) then only Christian burials.
So, grandpa might have been a viking but you're a local kind of thing.
Their trading though, and it wasn't always benign, they were known slavers, reached right down to Arabia. Christianity slowed down that trade eventually.
First European expansion I suppose