It's most likely incompletely combusted fuel getting into the hot exhaust and igniting. Afterburn/afterfire is a common phenomenon among highly tuned engines, especially those with anti-lag systems that incorporate ignition delay.
It can happen in naturally aspirated engines too. I built an RX-7 that used to lay out a tail of flame out the exhaust when lifting after a straight, and when downshifting. It seemed to happen most frequently heading into turn 7 of the original Thunderhill layout.
I imagine an ST/RS with a larger turbocharger and high flow (especially catless) DP would be more prone to this.
Check out this video of the old BMW F1 engine in qualifying trim. If you watch at around 10:20 and 12:05 you can see Berger making quali runs, and hear the pops and occasional flame bursts out the exhaust as he upshifts. The old Group B turbo cars used to do this all the time.
I wouldn't worry too much about it.
