According to the Wikipedia article Ordinance of Labourers 1349
During this outbreak [of Black Death], an estimated 30-40% of the population died. The decline in population left surviving workers in great demand in the agricultural economy of Britain.
Landowners had to face the choice of raising wages to compete for workers or letting their lands go unused. Wages for labourers rose and translated into inflation across the economy as goods became more expensive to produce. The wealthy elites suffered under the sudden economic shift. Difficulties in hiring labour created frustration. John Gower commented on post-plague labourers: "they are sluggish, they are scarce, and they are grasping. For the very little they do they demand the highest pay."
If the number of labourers and landowners was reduced by the same factor, it could only improve the life quality of everyone since more land was available per person. The only scenario I can conceive of which would be disadvantageous for the nobility, is that they were hit less hard by the plague than the workers. Thus there would be less workers per noble, which would be advantageous for the former and disadvantageous for the later. Is that indeed the case? If so, why were the nobles spared?