Pyrotechnician here. I've walked off gigs with my hands black from powder, smelling of powder, with so much powder falling from my bags that the security guards laughed about how "dirty" my bag was, got swabbed and went through without issue. Likewise, I have had colleagues travel with 12" mortar shells in their baggage without any issue.

I can only assume they're checking for deflagrating explosives (TATP, ampho, etc).