on turbo vehicles, it is more commom on MAF equipped vehicles if they utilize atmospheric blow-off valves. since the MAF meters air entering the engine, when the blow-off valve releases that pressure to the ambient air during a shift...it causes an extremely rich situation.

basically the engine thinks its getting more air than it really is.

ie. like fatbooty said, running rich is the main reason.

ive also heard of h22a motors spitting flame with the CAT removed...probably because of the overlap present at high rpm's.

~boom