Accidental mis-routing of a normal fart. Rather than blasting away from the buttocks, fart wraps around through the thighs, resulting in scrotal flappage. Flatulence then exits the front of body. . .
*Laying on back in bed and farting into mattress*
Girlfriend: Oh GAWD!! That stinks... Did you let another Boomerang Fart go?!?"
Boyfriend: *Grins awkwardly*