How can you explain why so many Germanic began invading the roman empire?

Answer 1

There was population pressure on the land in Europe north of the Roman Empire. Some two centuries before the final invasions, there was the beginning of migrations of Germanic peoples from southern Sweden and Norway, Denmark and northern Germany into central and eastern Europe which started squeezing the peoples who lived in central Europe. This led to the first invasions of the empire in the late 2nd century AD which Marcus Aurelius repelled.

In the 3rd century the invasions intensified and came from a large number of peoples. The Roman army repelled them but were put under such strain that this was period of military anarchy in the Roman Empire. In the west there were continuous raids by the Alemanni and Franks across the river Rhine area as peoples from further north were moving southwards, putting these two people under land pressure. They destroyed the countryside of northern Gaul. In the east the Goths had migrated from southern Sweden to Ukraine and created a large kingdom there. They carried out devastating invasions into the Balkan Peninsula (in southeastern Europe).

The final invasions in the 5th century were precipitated by the arrival of the Huns in eastern and central Europe. This squeezed peoples who lived in central Europe (the Vandals, Sueves, Alans and Burgundians) and forced them to cross the river Rhine together in 406 in search of new land. One thing about this invasion is that it was more than just military attacks. They were migrations of whole peoples, with often 10,000 people per group. Together they formed sheer hordes. Meanwhile the Roman army was busy dealing (successfully) with an invasion of Italy by an Ostrogoth king. The army was overstretched.