Probably it started with the tendency to add apostrophes to plurals that would seem awkward or ambiguous otherwise -- like, say, "straight A's" instead of "straight As." Which then got extended to numbers like "the 90's" for the decade (a slight distortion of what it should be, "the '90s"). Which then got extended by analogy to numbers in general ("highs in the low 90's"), and then to plurals in general. So like so many things in language, it started out as a justified bending of the rules for clarity's sake and just slid down the slippery slope from there.