Here's a relavant paragraph from Wright's book:

“One of the major problems with wads occur if they are loaded in such a way as to leave an airspace between the wad the base of the bullet. What is believed to happen is that as the powder ignites, the wad accelerates inside the case before the projectile moves. When the wad hits the stationary projectile it creates an area of high pressure and may cause chamber ringing of the area of the bullet base. The projectile effectively acts as an obstruction. This phenomenon is sometimes called the secondary projectile effect; the wad acts as a second projectile inside the case.”