Who were the most famous of the Germanic tribes that invaded and settled in Post Roman Britain and imposed their culture on the Britons and turned Britain into the England that the vikings (not Vikings) raided? The Angles and Saxons and Jutes, oh my!
Saxons was a very generic term for Northern German peoples, but any true Saxons invading Britain would have come from Saxony in northern Germany, Angles would have come from the Anglia peninsula and other parts of south Jutland on the Danish-German border, and Jutes came from northern Jutland in Denmark.
And there is evidence for cultural connections between the Anglo Saxons in England and other regions in Scandinavia such as Sweden. So apparently travel between the British Isles and Scandinavia, sometimes on a large scale, was not unheard of during the centuries before the viking (not Viking) raids began.
So I don't know what changed to start spectacular and recorded viking (not Viking) raids on the British Isles and western Europe in the 790s. Possibly Scandinavian traders in the British Islands noted that various places there were at the same latitudes as various places in Scandinavia, and thus that it was possible to sail directly across the North Sea at a particular latitude instead of going the long way around the North Sea. And that might have got viking (not Viking) raiders interested in targeting the British Isles.