I have a huge interest in Viking Age history but through my readings about the "Why did it start", there are many reasons that there were Scandinavians raiding places all over Europe but I have come across some explanations that I believe could be tied to White Supremacist narratives but am not sure if there is grounding in it or not. Here is the theory I want clarification on:
When Charlemagne came to power he sought to expand his empire and Christendom and invaded Saxony. Events such as the destruction of Irminsul and the Massacre of Verden (4000 men killed for not converting to Christianity) were events that created large tensions between Christians and Germanic heathens. Tensions arose more when Charlemagne banned trade between Christians and non-Christians (this point was told by a reenactment friend but I could not find documented evidence for this which if it exists, I want to know where from) eliminating previous trade with Scandinavians pushing them into poverty. The combined hatred for the foreignness of the Christian religion, what was done by Christendom to people of their faith (Saxony), and the fact that they were heading towards poverty because of the lack of previous trade partners pushed Northmen to engage in small ad hoc raids for supplementing their personal economy (and also showing their battle-worthiness for their society and their gods) but also made a point of targeting Christian targets, especially monasteries (the first recorded raid being at Lindisfarne monastery) because not only were they not well protected since there was the assumed notion of safety but also since they had a lot of wealth with relics and such.
The point of Charlemagne blocking trade against pagans was backed by another reenactment friend that said if you look at the Vendel period helmets, you see a lot of use of precious metals and number of gilded-items essentially cease to exist during the Viking Age. Thus you have the 3 Vendel helmets and only the one Viking Age Gjermundbu.
The other point that shows rising tensions between the Franks and Danes can be proven with the Danevirke and also the proliferation of Ring-forts around Denmark.
Thus how much can we say the (supposed) actions of Charlemagne were responsible for the initial raiding part of the Viking-Age? To what capacity would it be seen as a vengeance or defense of the Germanic heathen culture?
My answer to your question would be not much, although my knowledge is limited to the Frankish side but I will try to give you some additional context. First, the idea of Charlemagne invading Saxony to spread Christianity is a little bit more nuanced. The Saxons had been a problem for Charlemagne for quite some time. This was because the Saxons continuously raided into Frankish lands, so the invasion was not simply a power play by Charlemagne. Second, I haven't ever heard anything about Charlemagne banning trade between Christians and pagans, and there are several trade ports very close to Scandanavia (see Dorestad). Also I think its important to point out that the initial viking raids were directed at Anglo-Saxon England, not Francia.
In short, there is little that Charlemagne did that would "force" the northmen to raid, especially considering that the vikings were raiding pretty much simultaneously with Charlemagne. Its important to keep in mind that while the Verden Massacre takes place in 782, its not really another 20 or so years until Saxony is "conquered." Also I am not too sure what the linkages were between the Saxons and the Danes/northmen. Hopefully that at least gives you some information on what you have heard!