I think porn is driving it, inasmuch that it's in mainstream porn more often. It was the rise of gonzo porn and it's anything goes as long as it fucking attitude that allowed it to spread. A lot harder to fit gagging DT into a "plotline" on a Tera Patrick Jenna Jameson storied production.
When it was shown more it became popular and moved into the mainstream. Now on a lot of sites it on the general videos not even fetish.
Personally I think it's fantastic. Still struggling to find a south east pro$$ie that offers it regularly and to the same standard as US porn though. Hopefully AW will add "gagging" as a searchable option soon.
That or Lily-Mai will come out of retirement