It would seem to me that the term religion itself was a term given by an outsider to a group of people that may have been out of derision, or an identifying term.
I don't think that in and of itself it is that determining. There are a handful of major religions, but one cannot deny that when it comes to education and the fact that we have to have clear cut nomenclature to identify certain things, we get to the point that if the description does not completely fit the subject, then it is relegated to something else. Thus we arrive at pseudo.
If you do have groups that break off with different teachings, you get everything from dissenters, protestors, sects, heretics, cults, etc. It would seem to me that it is academia that uses the term pseudo for other topics including religion.