Up until the middle ages the history of the British Isles is littered with different groups migrating and settling the land despite people already living there; the angles, the saxons, vikings and so on. I recently read that Caesar's conquest of Gaul began with the migration of the Helvetii tribe across Gaul. This was described as a planned mass migration involving burning their original villages and moving hundreds of thousands of people to settle further across Gaul. Given the population has grown continuously since in the same land mass, what was the driving force behind all these mass migrations of people?
The exact reasons differ from group to group, but what all mass migrations have in common is their dependency on push-and-pull factors. This is as true today (Mexico-United States) as it was throughout history (Goths-Roman Empire). Push factors are the elements that contribute to a group leaving its current location, pull factors are the elements that contribute to a group settling in a new location. The exact elements involved can vary but for the motivation to be extensive enough for whole groups to migrate there needs to be a large imbalance between the prospects in the old location and the prospects in the new location, mainly in terms of economy and safety.
In relation to the mass migrations of the Angles, Jutes, Saxons and Frisians it is important to understand that their migrations were of a more gradual nature initially, as Roman foederati, only to snowball in order to fill the vacuum left behind with the Roman abandonment of the British Isles in he beginning of the 5th century. The most forceful push factor for these migrations was the increased local militarisation in northern Europe (leading to a deficit in safety), ironically augmented by Roman divide-and-conquer policies. As pull factor we have to consider the economic potential that was posed by all the unclaimed land left as a result from the aforementioned vacuum in contrast to the relative poverty of a colder climate (Scandinavia) and local terrain (bogs in the Low Countries pre-reclamation).
Considering the viking incursions throughout Europe, it appears to have been by and large an economic matter as /u/HankCo_employee mentioned: it is no coincidence that there are parallels between their location of origin and that of some of the groups involved in the earlier migrations to the British Isles. In contrast, the mass migration of the Helvetii is depicted as an act of desperation, with tends of thousands of them abandoning hundreds of villages, hinting heavily that they were in a situation of immediate danger. The problem we have here is that our main source on the matter, Caesar's Commentaries on the Gallc War, has to be considered as very self-promoting, and the numbers involved in the migrations are likely to have been inflated.
^(Sources used:)
^(Eeuwen des Onderscheids, Blockmans & Hoppenbrouwers, 2016 edition - Universty textbook on the Middle Ages)
^(De Oudheid, Naerebout & Singor, 2009 edition - University textbook on Antiquity)
^(Commentarii de Bello Gallico, Julius Caesar)
I concur with all the points Goiyon is so expertly and clearly making. I'd add that Peter Heather's 'The Fall of the Roman Empire' chronicles these migrations and the underlying push and pull forces well.
One other book somewhat against the grain is called Against the Grain, ha, sorry, by James Scott offers a very early look at some of the initial migratory patterns, e.g., pre-Sumer. His thesis is that the much lauded agrarian society and rise of agriculture is a myth perpetuated by those who created language as another means to retain their power. Scott points out some of the archaeological and anthropological evidence suggesting push factors away from the traditional, agrarian population centers.