Did the wood have cavities which you exposed while turning?
Did large pieces of wood come off as you were turning? If so what speed and type of tool?
Did the wood feel soft in some areas?
Is the issue just these pieces of walnut or other species?
Many potential root causes.
I have worked with a good amount of walnut, mostly in flat work, some in turning. I have not hard issues with walnut. I have had some boards with knots or obvious areas where the wood was soft or just not stable.
If the wood is solid (not punky -- starting to go rotten), this is most likely "tear out".
You don't say what tool you're using -- skew chisel, spindle gouge, scraper, carbide-insert scraper -- but tear out can often be reduced by having very sharp tools and taking very light "shearing" cuts (where the cutting edge of the tool meets the oncoming wood at a 45 degree angle).
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