Gaul it seems was the lynchpin for the Empire. It was primarily where the "Roman" element of the West Roman Army recruited from and was a major tax base for the Empire itself. With shenanigans ongoing in Italy and threats by the Alan's, Subei and such the locals pushed Constantine (Usurper) to revolt in their interests and when his forces took over Spain and failed to prevent the Barbarian Invasion his commander in Spain put forward his own candidate on behalf of the area.