The growth of feudalism in Europe during the middle ages was primarily caused by the?

Indirectly, the fall of Rome, and more directly, the threat of Viking attack. Viking raids during the Medieval period were common -- it WAS the Viking Age -- and what people do when they're under attack is run to the nearest fortified building. This building was either a church or the hall of a lord. Hence, in return for shelter and protection in case of emergencies, the lords asked for the loyalty and services of the peasants. Thus, feudalism.