I can't be sure from the photo, and I can't see the end grain, but it looks to me like you have gone a little too deep into the late ring, into what some people call lunar rings. They're appear sort of like mini early wood rings in the latewood. You really only notice them on thick ringed wood.
You can either just continue on, not going any deeper on those spots that show up, or if you have plenty of wood underneath, go to the lower ring of earlywood an be a little more careful when scraping.
In all likelihood you probably haven't gone deep enough to compromise the back ring.