sti's do it when people mod the BOV, the stock bov on an sti works just fine for up to 400awhp, people like to hear the cute little chirp between shifts, this causes an extremely rich condition because the air is recirculated not vented to atmosphere, so its released back into the exhaust and usually reignites.i got it with a full catless 3" exhaust off turbo, hks resognator. i found it undesirable and after doing my homework on BOV's took it off my sti and returned it to the stock one.but that is with a turbo'd engine. If a stock G is doing it i would imagine its due to some sort of tuning issue.
and it will usually happen after holding a gear at a high rpm for awhile and upshifting, not upon the initial downshift.