Normandy had been a base for Viking attacks on England since its foundation. The Saxons had a right to fight back.
The optimal time for an attack would be between 936 and 939 or 944-46. During these periods, England was ruled by effective kings (Athelstan and Edmund I); the Viking kingdoms in the North of England had been overthrown; France was ruled by a likely ally, Louis IV, who had been raised at Athelstan's court; and Normandy's northeastern neighbor, Flanders, was hostile to Normandy and could serve as a disembarkation point for the Saxon troops.
Normandy itself was ruled by William Longsword (until 942) then the infant Richard I. The latter's minority looks like the optimal time for a Saxon attack, but Flemish hostility peaked during William's reign, and it might have been easier to form a coalition of his enemies than of his sons'.