Above is a painting by Alphonse-Marie-Adolphe de Neuville depicting Charlemagne, ruler of the Carolingian Empire, converting an army of defeated Saxons to Christianity. This event, known as the Massacre of Verden, took place in 792 when Charlemagne defeated a Saxon army of 3000 men on the shores of the Elbe river. He then forcibly baptized them before ordering every prisoner to be drowned. Word of this display of brutality by the Franks reached the Danes of the time, and many scholars believe that it is no coincidence that the Viking Age began during the Saxon Wars. The Saxons were after all pagans who had previously been in regular contact with Scandinavians for trade. The link and association between northern germanic tribes and early Scandinavians is undeniable, and the forced baptism, if we are to believe the Royal Frankish Annals, was meant to warn the Danes of their impending fate should they refuse to convert.
Some historians go as far as to say the attack on Lindisfarne, which occurred in 793, may have been a retaliation for the Frankish conquests. The evidence lies with the account of what the Northmen did to the monks of that monastery. According to Alcuin, one of the primary sources for the attack on Lindisfarne, the Northmen dragged several monks to the beach where they drowned them. This is seen by many as evidence of the Northmen’s acknowledgement of Christendom’s aggressive expansion and barbaric treatment of non-Christians (including forced baptisms).
Unfortunately, there is no further evidence to suggest this hypothesis, and therefore it must be treated lightly. However, the people of the time appear to have been much more interconnected than previously acknowledged by academic circles, and those connections would have meant the Scandinavians saw the Christian Empire encroaching on their lands and rightfully retaliated. There are several other causative factors which historians associate with the beginning of the Viking Age, including a change in climate, the closing of Christian trade ports to non-Christians, as well as a population surplus in Scandinavia. It appears many factors played a role in sparking the conquests of the Vikings, but in general it can be heavily associated with the aggressive exploits of the Carolingians.