The first known viking raid in England happened in 789 when 3 ships from Hordaland, Western Norway landed on the Isle of Portland. I suppose that's the raid you're refeering to.

Since Sweden wasn't as close to the british isles as Norway and Denmark was, they did more raiding in Russia and the Baltic sea.

There are no 100% correct answers but I'll give an answer that is believed to be the most likely. Right before the viking age, there was a mass centralization in power in Sweden, Denmark and Norway, but Norway was hit the hardest. The petty kings ruling over larger areas than before taxed the inhabitants very harshly and seized power from the small chieftains. That caused a huge amount of cheftains and settlers to look for power elsewhere.

Another very likely theory is that the people living in Hordaland settled in fjords where there are alot of mountains. There was few places where farming was possible. Therefore, alot of desperate norse people turned to a life of plunder.

Sweden and Denmark did experience mass centralization, but Norway suffered the most from it. Sweden and Denmark didn't have alot of mountains and farming was therefore much easier there than it was in Norway.

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