I will speak of the Low Countries. I'm not sure how much this translates to other regions.

At the turn of the milennium, round about 1000, towns started popping up and growing at an impressive rate. These towns, unlike Roman cities, were focused mainly on trade. During this time money also became a more prevalent payment method.

Landowning lords initially loved this innovation and allowed their peasants to travel to nearby markets on a regular basis to sell their extra produce. More and more of these peasants were being paid in coins. That way they could start paying off certain duties they had to the lord, such as working his land several days a week, in cash rather than produce or labour.

As a result these peasants were emancipating themselves at a rapid pace. Although the lords themselves wanted to put a stop to this, they couldn't due to the fact that the peasants could flee to nearby towns if they did. According to city law such peasants would become free men after residing in the city for a certain amount of time. You would think the lord would go fetch them backed by an army, but that was also problematic. The counts and dukes of the Low Countries tended to support these cities to pressure the local lords and limit their power. This would however turn out to be incredibly problematic, as the cities of the Low Countries would become so powerful they in turn would have to be taken down a notch several centuries later after many conflicts with their lord.

The local lords in other words had no other choice than to continuously allow their peasants to emancipate themselves. This way the peasants started "owning" their own land. It was usually still owned by the lord in a strict sense, seeing as the peasants stillcpaid him rent, but the peasants had gained such extensive rights that it was basically theirs. E.g. they couldn't just be kicked out for no reason.

This topic is quite extensive and difficult to summarise but I hope this attempt was an intelligible summary.