It is used as a label to direct criticism at, alluding to a vague collection of commonalities shared by various postmodern writers, like social constructivism, cultural relativism, anti "tyranny of truth", and indeed anti "all order" as tyrannical, and therefore, as critics point out, none in particular. Think of Baudrillard's "revolt" against the consumer society in an "unforeseeable but certain" form, which comes with no theory of political organization or strategy like the New Left had in 1960-s, or Deleuze-Guattari's rage against the "machine" in Anti-Oedipus, which also goes nowhere. The phrase is particularly popular in pointing out that postmodernist ideology, for all its cultural "rebellion", does little to motivate actual social change in third world countries.
By the way, this is a typical use of "orthodox X" by those opposed to the "orthodoxy", whatever X might be. D'haen (1994), who is widely cited, see e.g. Migrant Cartographies, p.133, is explicit about it:
"I would maintain that "counter-postmodernism" functions as "supplement" to postmodernism in its "orthodox" definitions -- and perhaps here it is useful to insist that every definition only becomes "orthodox" in comparison with its successor. It "counters" orthodox postmodernism in putting its finger on the latter's complicity with what it purports to subvert or problematize, and thus "rewrites" -- in a move that duplicates the actual practice of much postcolonial and feminist literature -- orthodox postmodernism as one more form of the discourse of modernity, rather than as its transcendence.
With the same move, though, counter-postmodernism also "writes" the subjectivity, history, and language of those hitherto suppressed by the discourse of modernity as emanating from western bourgeois society. As such, it makes this discourse also accessible to those traditionally excluded or repressed by western modernity. Ironically, by thus marking the end of modernity as the exclusive property of hegemonic western man, and the advent of modernity also for the hitherto repressed, counter-postmodernism may well be the only truly "post-modern" reading of postmodernism in that it posits the transcendence of orthodox modernity, and the attainment of an-"Other" modernity."
Larsen in Postmodernism and Imperialism used the expression even earlier (1990) for the same purpose:
"Both Jameson and Said -- the former far more openly and forthrightly than the latter -- violate central tenets of postmodernism, of course, insofar as they posit the existence of a marginal consciousness imbued with "presence" and self-identity." That is, they appear to justify an orthodox postmodernist counter-accusation of "essentialism"... Does the move to, as it were, found post-modernism's anti-foundationalism in the rebellious consciousness of those marginalized by modernity alter orthodox postmodernism's reactionary character?
[...] I dwell on this because I think a truly critical assessment of an "anti-imperialist" postmodernism, as of orthodox postmodernism, requires a prior recognition of the essentially parasitical dependence of such thinking on Marxism and particularly on the crisis within Marxism -- a dependence which, as we have repeatedly observed, postmodernism must systematically seek to erase."