After ignition, the pressure between the chamber and the case is considerably higher than that of a delayed unlocking of a tilt barrel designed pistol. Add in the barrel length and it increases it further.
All that gas wants to escape asap, some is going to take a short-cut to get out. And that bit comes out back toward the firing mechanism from outside the case.
All my blowback semi's get dirtiest of all. I'd consider it as normal operation.