Quoted:
So let me gets this straight...
You took a cup that was larger and replaced the factory one, so in the end your wax slugs were heavier then the original shot in the birdshot? Was there a noticeable amount of recoil?
That will do it. I guess you are not a reloader because a reloader would know, the heavier the projectile the less powder it takes. So you significantly increased the weight of the projectile and kept the same amount of powder. Its also worth it to note that birdshot uses somewhat flimsy casings that are not meant for higher pressures.
In red...
While you're possibly correct a higher chance is having a space in the shell where the powder sits. He did exactly what Cowboy Action guys do when they routinely blow themselves up: Underloaded a charge leaving a gap. The powder is supposed to burn from rear to front pretty uniformly. If it's not compacted, you can get more than one place the powder is burning from, exponentially increasing chamber pressures. (You can easily double or triple them)
Ever wonder why they put oatmeal or whatever in their underloaded cartridges to keep the powder pressed against the primer? Now you know. Whenever I hear about a newbie to the sport talking about how they'll pull a bullet, dump some powder, and replace the bullet, I get out of Dodge when they're going to fire. They do this to make it easier to fire fast and on target.
OP: The reason you experienced what you did was because you replaced the wad and replaced it with another one without it compressing the powder so there were no empty areas.