Feudalism began as a result of the contact between the Roman world and the Germanic people. Initially the Germanics had a settled agricultural economy with a pronounced pastoral component. As pastoralists, private ownership of land was unknown. However contact with Roman luxury meant a need for accumulated wealth. Rome fostered this need by political alliances. Having settled tribal units on it's borders meant it was easier to defend those borders. Equally it recruited a high proportion of Germanics into its army. and often these men gained high office. The border tribes began to make war on other tribes and part of the booty was slaves which could be traded to the Romans. Leaders began to gather "retinues" about them, men on whom they could call in times of war. I'll go off on a complete tangent at this moment toi illustrate this with the giving of rings in Beowulf.
But the system ws put under even more pressure by the arrival of the Huns. Now more than ever tribes faced a need to protect what was theirs and the only way was by "warlords". The beginning of a warrior aristocracy had arrived.