More surprisingly perhaps, the rise in Normandy’s fortunes came about as a result of being dragged into near economic and social collapse in the wake of the fall of an Empire.
Gaul (originally populated by Celtic Gauls and our freind Asterix) had been a province of the Roman Empire centrally governed by Rome and the Divine Emperor, but by the time the last Emperor was deposed in 476AD, the Western provinces had become money pits. The fall of the Rome was not a sudden event, it was more of a slow disintegration. The cost of civil war and constant insurgences by Germanic tribes into Western Provinces and the resulting loss of income from trade, were part of a fatal economic decline that not even devaluation and high taxes could fix.
The Salian Franks, invaders and pirates from the Netherlands, were already a presence in Roman Gaul, originally as Laiti (permitted foreign settlers), then (from 357) as kingdoms supporting the Roman army. In later years they became increasingly difficult to control and by 509AD Clovis I was able to expand his territories and create a new dynasty, the Merogivinians in the new empire of Francia (France).