The serfs were required by English commonlaw to remain in the service of their Lord. They couldn't leave his land. Failure to maintain the loyalty of one's serfs could cause them to break the law and leave, looking for other work, which often meant robbery and chicanery. Thus they became villains, meaning literally, land-bound peasants who had left the estate (villa) of their liege Lord.
Sounds like no more than a guess. Then again, an impoverished baron would more likely seek to repatriate the taxes originally paid to the king by himself and others instead of preying on his neighbors, now, wouldn't he? Besides, Robin would have joined forces with the other barons (and respected them) to defeat John at the Battle of Runnymede in 1215, because they ALL hated his guts, right? WHY do you think they forced him to sign the Magna Carta upon his defeat, giving birth to the fledgling idea of the rights of men other than the monarch?