Well, they did have a technological advantage in the form of viking longships. These ships were long, shallow bottomed, flexible ships that were both graceful as well as being some of the fastest ships in the viking's geological sphere of influence. They were highly efficient in the sea as well as in the small rivers and fjords of Scandinavia and their shallow hulls allowed them to travel up mainland rivers, even reaching as far as Paris, France before the end of the viking age. The ships also allowed for long, fast voyages along coasts carrying vikings as far from Scandinavia as Italy, Turkey, Russia, North Africa and Canada.
Another piece of the reason they were so successful was that they often targeted under-manned monasteries, churches and small villages. 8th to 12th century England was not united by any means; you had North Umbria, Mercia, East Anglia, Wessex and all the smaller states within those areas that were not always at peace with each other, requiring fighting forces that were not seen to be as needed on the north and northeast coasts of England and modern Scotland.
Now we come to combining these two factors in viking tactics. Vikings were raiders, at least in the beginning, and were not setting out to conquer lands and steal fortifications as in your typical medieval battle. They use a hit-and-run style of raiding that left their victims little to no time to call for aid. They would spend their winters at home preparing their ships, weapons and bodies for the summer raids and after the spring crops had been planted they were off in search of the most plunder they could bring back with the smallest amount of risk involved. To a viking, it didn't matter if you were a soldier or a monk, if they engaged you in a fight and you lost, they were entitled to what you owned as they considered this a fair fight. So, in that way, they may have also had a psychological advantage as well. Other monks and god-fearing men heard account of these ruthless demons (some letters from monks who escaped the vikings survive these encounters) and fear and infamy about them spread through the British isles.
If you’re interested in further reading I highly recommend “The Viking World” https://www.amazon.com/Viking-World-Routledge-Worlds/dp/0415692628
If you’re interested in reading a letter written about the vikings by a monk whose monastery was attacked by vikings, Yale has an online transcription available here: https://classesv2.yale.edu/access/content/user/haw6/Vikings/higbald.html