Some explosives, like C4, can actually be safely burned. To set them off, a detonator is required. Explosives from WW2 probably have similar properties, as it makes producing and handling huge warheads a much less dangerous activity. Of course, there might always be impurities in the material that makes them behave much more unstably, both from production and because of corrosion and chemical degradation over the ~80 years that have passed.