Leading is a general term, but can come from several mechanisms. The base of the bullet can be melted by the hot powder gasses and lead the barrel, the lube can fail to do it's job either due to an undersize bullet or insufficient lube causing the bullet to lead, or the rifling can be too fast for the bullet hardness and the rifling can shear the bullet causing leading. Not to mention the peak pressure the powder generates and velocity which play into the mix.
If you have a bullet that is just on the verge of leading and you are using a powder that is squirrely, i.e. causing pressure spikes. Then that occasional round with a pressure spike can cause leading.
