The assumption is that creating more rationality contributes to creating more order, which will make society function better. Because modernist are about the pursuit of ever increasing order, modern societies are constantly on guard against anything and everything regarded as disorder. The ways that modern societies go about creating categories labeled as order or disorder have to do with the effort to achieve stability. .
A theorist Francois Lyotard (the philosopher whose works Sarup describes in his article on postmodernism) equates that stability with the idea of "totality," or a totalized system. Totality, stability, and order, Lyotard argues, are maintained in modern societies through the means of "grand narratives" or "master narratives", which are stories a culture tells itself about its practices and beliefs. A grand narrative in American culture might be the story that democracy is the most enlightened form of government, and that democracy can and will lead to universal human happiness. Every belief system has its grand narratives like for instance Marxism. The grand narrative is the idea that capitalism will collapse in on itself and a utopian socialist world will be created. I guess you could think of a grand narrative as an ideology that explains an ideology (like Marxism).
Postmodernism then is the critique of grand narratives, the awareness that such narratives serve to mask the contradictions and instabilities that are associated with any social group or practice. In other words, every attempt to create order always demands the creation of an equal amount of disorder, but a grand narrative masks the construction of these categories saying that disorder really is chaotic and bad, and that order really is rational and good. Postmodernism rejects grand narratives and favors "mini-narratives," stories that explain small practices, local events, rather than large-scale universal concepts.