The Saxons, Angles and the other Germanic tribes along the European coast probably right up to Denmark had a very long tradition of raiding, going back to before the Roman invasion of Britain (in fact the famous Viking longship is basically a larger version of the Germanic ship, with a keel), they were well known in the Roman Empire for raiding along the North Sea coasts as well as into the Mediterranean, and the Roman defences and fleet along the Rhine and the British 'Saxon Shore' were there to try to stop them.
To my mind, it is less about 'rites of passage' or 'population pressure', and a lot to do with private enterprise. Consider a situation where a small consortium comes together and raises a bit of capital (perhaps borrowing from friends/money-lenders) as a speculative business opportunity - only instead of trading, it's raiding! The idea being that they use the profits from piracy/raiding to repay their loans and invest in bigger/more ships and crews - it would be very similar to the standard Traveller trading mini-game, but with a lot more violence, whether implied or actual.
Similarly the 'Barbary Corsairs' who raided around the Mediterranean, Europe and North Africa were also in it for the profit.
