Dr. Strangemonkey said:
I think it would be difficult to dispute the intermittent appearance of coastal raiders throughout the history of the North Sea, but I also think it would be difficult to say that the Viking invasions represented a new spin of an always present phenomena.
There must have been a fairly extensive period in which coastal raiders were a much reduced threat in order for the British system of Island and peninsular monastaries to have developed. Communities of people who have largely disavowed violence and commerce have to be built for defense. The fact that these communities were among the first to suffer heavily under the Viking threat indicates that coastal raiders were a threat that the generations who had built the monastaries were completely unaware of.
I do agree, however, that the Norse population explosion does seem to me to be a rather unnecessary theory. Sometimes a people just develops the right combination of technology and ancyness at the right time to go Viking.