In what ways could an ordinary free man become dependent on the nobility? Why did this happen?

People of various incomes who were not in the service of the king were considered free. As a result of internecine and external wars, as well as various taxes, a free person fell into dependence on the nobility. In the Frankish kingdom, for example, there was a commandment, it led to the loss of freedom. The poor promised to serve the feudal lords, becoming dependent for debts and becoming hereditary slaves. Horse warriors also fell into bondage. For military service, they received land (benefits) and became vassals, dependent on the terms of ownership.