The Frankish Annals give a glimpse into why the Vikings attacked Britain, and later Normandy. We know that Charlemagne interfered in the Danish civil war and that while waiting for the Danish to attack, they took up defensive positions and launched a massive naval assault on Frisia, modern day Holland. We know this naval assault forced Charlemagne to abandon his Danish campaign, retreat and spend a year marching along the northern coast to prevent further naval strikes, what would later be known as viking raids.
We know that Danish warriors are recorded in these annals as saying some of their kin were already engaged in an invasion of Scotland. We know that it was around this time that the first Norwegian assaults on Scotland were happening. We know that Denmark controlled much of southern Norway at this time.
We also know that Widikund was initially driven from Saxony and took refuge in Denmark. We know he did not return for several years. What he did during this time remains a mystery. We know that Charlemagne was importing Anglo-Saxon priests and monks from England to forcibly convert Saxons, their distant relatives leftover from the original Anglo-Saxon invasion of England hundreds of years prior. This may have been considered adding insult to injury for the Saxons and any northern sympathizers. The initial viking raids in England were all monasteries.
There is no reason to believe in some great Scandinavian alliance, but nor is there any reason to believe they were dumb as a box of rocks. They were sea traders and would have some knowledge of the Frankish conquest of Frisia, which had formerly been pagan. Witnessing the Frankish invasion of Saxony and the ambitions of the Franks in Denmark may have been an awakening for them. No alliance would be necessary to form viking raids among a diverse Scandinavian population much in the same way many diverse Europeans later banded together for the Crusades (some of which were merely collections of civilians, sometimes children), or how modern day antifa members band together from across the nation to converge on a single location like Charlottesville. We know the Franks originated from the northern lands around the same time as the Lombards, and their history may have been better known by Scandinavians of the time who may have considered these forced conversions a betrayal of the worst kind.