I've experienced this too when switching to boots with a higher heel pitch.
My explanation is that, when skating backward, something (I suspect this is what your question is about, but I don't know exactly what the 'something' is) about your position and centre of gravity causes your weight to naturally plant farther forward on your blades.
The higher heel pitch has this same effect, puts your weight farther forward on your blade whether you're going forward or backward, but you probably notice it more backward since the effect is basically doubled.
Add some insoles that boost your heel slightly to all of this, and you'll notice sudden and dramatic toepick scraping going backward!