A prayer book seems like an odd place to stage a theological revolution. It is, after all, mostly the same words a congregation has said for centuries, printed again. That is precisely what makes Mordecai Kaplan's 1945 Sabbath Prayer Book such a pointed document: he kept the structure and much of the Hebrew of the historic Sabbath service intact, and edited what a worshiper was actually being asked to affirm while saying it.
Kaplan's target was chosenness, the traditional liturgy's repeated assertion that the Jewish people hold a unique, divinely selected status among nations. He considered that claim, along with several related traditional doctrines, no longer credible to a modern worshiper, and Reconstructionist theology held that Judaism should be understood as an evolving religious civilization rather than a fixed set of supernatural claims. So instead of writing an entirely new liturgy, or abandoning the traditional service altogether, he revised the specific passages that made those claims, leaving a service still clearly recognizable as the Sabbath liturgy but no longer saying, in the worshiper's own voice, what Kaplan believed it should no longer say.
That editorial choice, changing the theology by changing the text rather than the ritual, is exactly what the Orthodox rabbis who burned a copy of the book in 1945 objected to. A new sect writing its own liturgy from scratch could be ignored. A revision that kept the familiar service intact while rewriting its claims read, to them, as something closer to a hostile takeover of the thing itself.