I tend to think this is caused not necessarily "lean" or "rich" but both in quick succession due to lag in sensors. Your MAP or MAF sensor isn't instantaneous, so when you suddenly go from 100% to 0% throttle there can be spikes. I would tend to think you're rich at high throttle, and a sudden throttle reduction to 0% can cause the injectors to turn off, pump some of the leftover air in the intake manifold (pure air, ~21% oxygen) into the exhaust where there is still some rich mixture, and the super hot turbine wheel or other hotspot in the exhaust will trigger combustion.
I wouldn't sweat it. Your car always did this, you just didn't really notice due to the factory catalysts, mufflers, and resonators.
2009 BMW 135i 6MT Sport, AFE intake, Cobb AP, Apex 18x8.5+9.5, 255/275 PSS