Well, for one the plague ravaged the military in Constantinople, which King John sought to re-enforce for various reasons, but largely because he got his ass kicked in Normandy.
Raising money to to join the crusade, he oppressed the barons with scutage (tax paid by a knight in lieu of military service), the selling of women (John made a regular traffic in the sale of wards, maids of 14 and widows alike), forest stealing, and taking children hostage for ransom (he slaughtered 28 sons of Welsh hostages).
These abuses pissed off the barons sufficiently.
Second, people forget much of the MC had to do with Jewish rights -— the Jews became pretty strong numerically and financially at the time because the Jews were largely untouched by the plague (we were in league with the Devil, you know -— or it could be keeping Levitical cleanliness kept rats at reasonable numbers).
Third, simple balance of power of rural (less plague) vs. city (more plague).
Blow smoke somewhere else.