I regard the barbarian pirate fleets as a side-effect of the EB Sweboz Confederation dominating a vast area of Germany and Scandinavia. The German tribes, instead of constantly fighting each other as they did historically, are all allies and living peacefully with each other. The Sweboz king won't let his subjects kill each other anymore, which really upsets many of them - but the Sweboz royal armies are too strong to be defeated by an internal rebellion.

So the many, many German warriors who don't want to be peaceful farmers and don't get recruited into the Sweboz royal army need another outlet for their aggression. Many of them rebel, but instead of trying to defeat the Sweboz as a land army, they run to remote coastal villages and become sailors and pirates - preying on the Sweboz themselves as often as not. There will be a great many pirates originating from Norway, Denmark, and northern Germany.

Basically, think of it as the Viking seafaring culture being created 1,000 years early!