Only because the first recorded use in writing was in a Protestant polemical piece. That doesn't mean it's not true. I would think that would still be the best guess - most people in those times would have thought it sounded somewhat like the Latin Mass. Whether it was started by an irreverent Catholic or Protestant, a send up of the Mass seems a logical origin for it. There are documented alleged cases in the 16th century and earlier of profane and cynical priests doing just that kind of thing - saying nonsense phrases instead of the right Latin words and having a joke on the ignorant peasants who thought they were getting the Sacrament according to the rites of the Church when all they were being offered was...hocus pocus.