The use of barbarians to bolster the numbers of the roman military and in particular, allowing them into the officer class was one of the causes of dissention in the roman army. It started early on and they became increasingly reliant on them.This is what Zosimus wrote: "The barbarians above the Rhine, assaulting everything at their pleasure, reduced both the inhabitants of Britain and some of the Celtic people ( the population of Celtica, the north-western part of Gaul) to defecting from Roman rule and living their own lives disassociated from the Roman law.
276 - 279 Probus defeats alemannic insurection around the Rhine and along with their leader Igillus "sent [them] to Britain, where they settled, and were subsequently very serviceable to the emperor when any insurrection broke out."
306, Crocus, who is described as "Alamannorum rege," plays a key role in the accession of Constantine at York.
372 Valentinian sends the Alamannic Fraomarius of the Bucinobantes (an alamannic canton), along with other Alamannic troops commanded by Bitheridius and Hortarius, to Britain.
The Bucinobantes were an alemannic canton around Mainz. They were troublesome from early on and sending them to Britain was one way of dealing with them. Around 260, they were raiding freely in Roman Gaul:
Many hoards of stolen goods have been found, one of the most spectacular in Neupotz, found in an oxbow lake of the Rhein.