To put what other's have already answered in a slightly different way, I'd say it's comes from a crude understanding of Marxism and ideology. These people read somewhere that class-based action to change material conditions is the only way to fundamentally improve society so they imagine some "pure proletarian revolution" situation where the labour struggle is all that matters and we don't have to worry about issues that affect the masses more broadly (i.e. racism, heterosexism, transphobia, etc.) because they will "spontaneously" or "automatically" solve themselves at some later time, and doing so before-hand is impossible, a waste of time better put elsewhere, or some other nonsense.

It's a mechanistic, un-dialectical application of the base-superstructure relationship and how ideology and hegemony work in general that must be struggled against, and which Marxists have been struggling against for over a century. Lenin writes in What is to be Done? about the importance of not confining the struggle to a narrow, labour and class basis and that communist must be at the vanguard of every social issue that affects the masses.