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My .45s do that, even in my nice tight national match chamber. It's from exactly what you said...burning powder leaking back from incomplete obturation. Basically the pressure is too low to completely seal the brass to the chamber for whatever reason. Maybe the case is a little thick, the chamber is a little generous, the brass a little stiff, or whatever. Doesn't seem to hurt anything.
In rifle brass I see it as a low pressure sign and try to keep my loads above the level where I get bad soot so long as I'm not near max loads. My powder puff .30-30 loads do it with light bullets and light loads.
You don't want to push the pressure in the .380 since in a blowback action, you want the brass to be able to start moving back and ejecting while the bullet is still in the barrel.
It polishes right off in the tumbler.
-J.
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