Well I've answered a question similar to this before, here

However I'd like to go through the reasons that you listed and unpack them a little bit.

Part 1

Vikign Age is traditionally marked as starting in 793 with the raid on the island of Lindesfarne. Now if this was a response to the Saxon Wars waged by Charlemagne, they're in the wrong neighborhood. Lindisfarne is located in England, at the time in the Kingdom of Northumbria, quite a ways away from the Franks who were pressing onto Scandinavia. Now while polities in Denmark were clearly concerned with the encroachment of the Carolingians and later the Ottonians close to their territory, its hard to see the start of raids on England as a direct response to Carolingian aggression.

Further more there was no unified authority that could carry out such a policy of retaliation. Germanic heathens utterly lacked the infrastructure of religion, there were no dioceses or positions of authority that could compel different kingdoms to a course of action. Our understanding of Norse religious life is far from complete, but the existence of religious authority that transcended political units was clearly not present.

There were indeed missionaries active in the area at the time, but they were making very little headway into converting the populace. While some of the slightly more urbanized areas of Scandinavia might have housed small Christian populations the vast majority of the populace was still pagan, as were the elites of society. Launching raids in response to pretty tepid missionary activity is not only a little disproportionate it is not supported by the textual evidence.

Part 2

Population booms are a traditional explanation for mass migratory movements, and after all it makes a certain intuitive sense. Too many people, some people need to move, hence large movements of people. Going back to Caesar at least they are a standard go to explanation for why one society starts expanding into new areas. The vikings are no exception and the idea has bee floated before. However as an explanation for the Viking Age it has been utterly abandoned.

People quite simply don't need to up and move due to overpopulation. They move around for a plethora of reasons, economic opportunity, fleeing aggression or persecution, as a part of military campaigns, and all of those certainly played a part in Viking expansion, but overpopulation almost certainly did not.

Land was certainly in hot demand in Scandinavia and settlements across the North Sea were founded, there is no evidence to suggest they were set up to alleviate the pressures of a rapidly expanding population. Indeed, according to Jenny Jochens's Women in Old Norse Society one of the greatest concerns in northern households was having too many mouths to feed, and that infant exposure was one such response to such pressures. Hardly indicative of a society that is becoming overpopulated.

While there were certainly people who were going to lose out on inheritance who took to the seas to make a fortune, this is not necessarily indicative of a lack of land available or overpopulation. Rather we should examine these people as a product of their society that valued and privileged martial prowess and the relative ease of economic advancement through raiding and other activities such as trade or mercenary service.

Part 3

Viking raids started as I stated above in 793. At this moment, England was divided into a wide variety of kingdoms, with the regional supremacy varying between a variety of kingdoms, at this time Mercia was ascendant. However calling this a de-centralized area is misleading. While there was no unitary kingdom of England, these kingdoms that existed were in the midst of their own centralization. Robin Fleming argues that these kingdoms were starting to really develop economically, and subsequently politically, through the trade emporium system that the viking age helped to end.

Meanwhile on the continent it is extremely hard to argue that this was a period of de-centralization in Europe. Charlemagne was crowned Roman Emperor in 800, a scarce 7 years after the attack on Lindisfarne and under his rule and that of his son, western Europe was more united and centralized than at any point since the fall of the Roman Empire in the west. While later Carolingian divisions would certainly play a factor in the continued success of the Vikings, there is no reasons to suspect it was a formative factor.