What you are seeing is called torching. For some reason, fuel is in the turbine and exhaust (leaking fuel shut-off valve, residual fuel from the previous run, ignition not coming on as quickly as the fuel, fuel control fuel scheduling) and when ignition occurs, this fuel burns off in a flash or torch. Generally, this is not a technical problem, but if someone who does not understand what is happening sees this, all hell can break loose. Planes have been evacuated, fire trucks called out, etc. The Keystone Kops would be envious of the melee that results. If the torching is excessive (it won't stop), the normal procedure is to turn off fuel and ignition and continue to turn the engine with the starter (this is called motoring the engine); this will normally cut off the fuel and ignition source and allow whatever residual fuel is remaining to burn off or blow out of the exhaust. If a fuel shut-off valve is leaking, motoring won't fix the problem as more fuel is feeding the fire. Then you have to shut down and call the fire folks. Unfortunately, in my experience, most of these peole do not know how to fight a tail pipe fire and the result is an unnecessary engine change due to fire firing foam being shot everywhere!! Groan!!