See post #17...labor was so scarce that the laborers were able to begin negotiating contracts and rights in return for their labor and loyalties. This grew into a tradition of negotiation and eventually the Magna Carta was born...or so I've seen speculated.
I think Magna Carta resulted more from the English having to live under what they considered to be a foreign king and nobility. I doubt John even spoke English.
No scutage or aid...
Magna Carta - 1215
Black Death - 1348
No connection.