Its unburnt fuel getting burnt in the hot exhaust. Its normally caused by valve overlap which is where the exhaust valve is partially open and allow a little gas to escape, normally an exhaust is tuned so that the resonance in the exhaust pipe will push some of the unspent fuel back into the chamber, where it should be, before the valve fully closes. When the engine is running hard the gas flow is not as efficient as at lower revs so the fuel stays in the exhaust and gets burnt creating flames out the exhaust. Turbo exhausts tend to be more free flowing also so they are more prone to this at high revs.
Generally the more tuned your car is for performance the engine tolerances are smaller so this happens more during hard driving. As somebody said above if the cars running a little rich that will also cause it, the ECU will try to compensate for this but depending on the cars setup, sometimes it cant. It wont hurt your car if its rich but it will foul plugs and sensors if its like that for a long time. But since as it just happens when your going hard its probably not an issue.