If it's the scene I'm thinking of, which takes place at the start of the film, the fire hazard is the whole point. The scene takes place during the Battle of France. During this period, the Allied forces in France were forced to retreat in disarray by the rapid advance of German forces. This included the RAF's forces in France, who sometimes had to evacuate airfields shortly before the Germans arrived. Evacuating an airfield was a difficult business. Airfields aren't just where aircraft take off from and land on, they are where aircraft are maintained and repaired. If an airfield needs to be evacuated in a hurry, then serviceable aircraft can just take off and fly to the next airfield. The service personnel can be carried there in trucks or transport aircraft. Moving aircraft that are under repair, though, is harder. They can't fly, and they're too big and heavy to carry on trucks. Abandoning them to the Germans isn't a good option either; letting them recover the aircraft might help them learn better tactics to use against them. Setting fire to them ensured that the Germans wouldn't get their hands on them, and it wasn't much of a loss since the aircraft would be abandoned anyway. As an example, the RAF's official history mentions No. 73 Squadron doing this to its disabled Hurricanes at Nantes prior to the final evacuation from France.