During the plague the economy in most places ground to a halt. This meant that manor lords/dukes/nobles that owned land nearly stopped producing food and taxes for their respective governing bodies, causing a slow decline in their power and status over a few decades. Basically, they weren’t contributing to the crown so they just sort of became less important/powerful over the years.

Additionally, with the Crusades, travel and general knowledge of the larger world increased massively all around Europe. Many new trade routes were established, and centers of trade grew until they were large and bustling cities. Now with most manor lords holding less important status and with most of them completely broke, peasants were allowed to move to centers of trade to find work and slowly buy their way out of surfdom.

In England, the invention of the longbow and its massive advantages over the recurve and crossbow made archery a less specialized skill, allowing almost anyone to be trained up as a decent archer in a few weeks. As an archer myself I’ll attest to this: longbows are FAR more forgiving than hunting/standard bows due to the fact that an error in wrist movement or placement is less relevant than on a smaller bows. Most English armies were converted to center around massive longbow vollies to weaken enemy armies before moving to up close combat. This made knights, who were their own noble feudal class in most places, far less valuable on the battlefield overall and eventually led to the the decline in power of the knightly classes all over Europe.

So with all of these combined, it wasn’t that surfs and peasants were propped up but that the nobility was being taken down a few pegs, causing the proverbial “playing field” to move closer to being level.