Is there an explanation for why westerners who follow an anti reactionary ideology have a loud purist minority?

Maybe a combination of something like the importance placed on forms while neglecting substance, and something like this:

The problem here, in short, is elitism. Unchecked, presumed to have been neutralized in some way by the adoption of a counter-cultural ethos, it festers. The way to solve it, however, is not to shy away from studying or exposing bourgeois propaganda, but to delve even deeper and radicalize our understanding of it.

I think an important distinction to make here is that between the directly oppressed who might just in the earlier stages of class consciousness and class struggle sort of replicate the form through which they are oppressed, and those who are part of the privileged groups but claim to support anti-colonialism, anti-imperialism, etc.

I would also say that due to their often more privileged position, these types, due to their remaining idealism tend to think they have all the answers, and that they know better than others. A sort of western chauvinism which takes its own answers to be the absolutely correct everywhere else. Just because they proclaim, or maybe even truly believe in these causes, they cannot look past their own chauvinism and continue to absolutize their point of view.

Losurdo describes chauvinism, in regards to nationalism and internationalism, but I think his formulation can be extrapolated onto other forms of chauvinism as well:

The repression of national particularities in the name of an abstract ‘internationalism’ facilitates things for a nation intent presenting itself as the embodiment of the universal; and this is precisely what chauvinism—in fact, the most fanatical chauvinism—consists in.

Losurdo also ventures into an analysis of similar phenomena to what you describe and characterizes them as populism which stems from a reductive reading of the theory of class struggle (among other things) which limits it to just oppressed vs oppressor, and tends to lead to putting the oppressed identity on a pedestal without much analysis. He deals with it in chapter 13 of Class Struggle if you want to read it all, which I definitely recommend.

This is a further expression of populism: moral excellence lies with the oppressed who rebel and those who offer help to the oppressed and rebels. But once they have won power, the latter cease to be oppressed and rebels and forfeit their moral excellence. And the one who, by virtue of aiding them, basks in their moral excellence also finds himself in serious difficulties. This is a dialectic already analysed by Hegel in connection with the Christian commandment to aid the poor, which manifestly assumes the permanence of poverty.