In 407 AD Contantine III had taken a significant part of the Roman Army establishment in Britain across to the continent to further his imperial ambitions. Following which in 408/409 AD there was a serious and destructive sea-borne attack on Roman Britain by the Saxons.
As the Gallic Chronicle of 452 states for the period,
The Britons were devastated by an incursion of the Saxons
Which led the Britons to revolt against the Roman authorities in Britain.
All that we really know of these events is based on Zosimus
The barbarians above the Rhine, assaulting without hindrance, reduced both the inhabitants of Britain and some of the Gallic peoples to defecting from the Roman rule and living their own lives, disassociated from the Roman laws. Accordingly the Britons took up arms and, with no consideration to the danger to themselves, freed their own cities from the barbarian threat; likewise all of Armorica and other Gallic provinces freed themselves in imitation of the Britons, ejecting the Roman magistrates and setting up a constitution such as they pleased.
Zosimus further makes it clear who is responsible for these events
Now the defection of Britain and the Gallic peoples took place during Constantine’s tyranny, the barbarians having mounted their attacks owing to the carelessness in administration.
And that, unfortunately is the sum total of our knowledge. We know nothing of the nature of the rebellion; nothing about who they were or what their objectives were. Edward Gibbon wrote that the land
was ruled by the authority of the clergy, the nobles, and the muncipal towns
but admits that this is based solely on conjecture
We can surmise that they must have been led by the local Romano-British nobility, clearly
frustrated by the inability of these Roman magistrates to protect them and perhaps the desire to free themselves from the payment of imperial taxes. It is unlikely that there was unanimity amongst them; the rejection of Constantine III's officials must have seemed to some as a bid for political and military independence from Rome, to others merely a means of demonstrating their loyalty to Honorius, the legitimate ruler of the Western Roman Empire, and a necessary prelude to the readmittance of Britain to the commonwealth of Rome.
It is likely that these differences persisted long afterwards, and formed the basis of the internal political struggles that dominated Britain in the first half of the 5th century.
It is also worth pointing out that irrespective of what troops Constantine III had withdrawn to support his bid for empire we can see that Roman Britain was far from defenceless; they "freed their own cities from the barbarian threat". Sufficient military resources therefore existed, whether in the form of legionary garrisons or locally raised troops to effectively counter the barbarian incursions.
Part of the Sub-Roman Britain project, where sources are detailed.