Bullseye is a double-base powder with lots of energy content and little retardant. It needs little more than a spark to ignite it. Slow single-base powders or heavily retarded double base ball powders need pressure, not just ignition heat, to burn well. Occasional detonations of light loads of slow powders was well debated in the 50's and 60's and is now an accepted fact of interior ballistics. They are easier to understand given the facts above, and the fact that most energetic materials have two burn rates, a deflagration rate and a detonation rate.
Explosives are self-consuming at their detonation rate.
Most energetic materials can be consumed at either rate, given certain circumstances. Dynamite lit with a match will produce a blazing bonfire, but no explosion. Expose it to the concussion of a blasting cap and it will detonate. Detonations of small charges of slow powder work the same way. Often pressure waves move the powder around and pack it against the base of the bullet.