Defintie mis fire and as said above most likely a "dud" round. Always wait at least a minute when this happens incase the misfire turns into a hangfire. Open the gun and quickly remove the round. I've seen lads do this very slowly and frankly cannot understand why. You want it out fast incase it does go off.

Did you check the round afterwards? Did it seem lite?

Sounds like a squib round. It's also known as "all pop and no kick" which seems very relevant to your situation.

It's a faulty primer or more accurately the failure of the primer to ignite the propellant. Effective the round is dead. The primer has been expended so cannot "all of a sudden fire", but to be on the safe side i'd cut the cartridge and dump the shot/propellant out of it. Then dispose of the shell/case. It's rare in that i've never seen it or had it happen to me, but have heard others have it happen to them.

The only saving grace is it's a shotgun. Meaning it's not a single projectile that can get stuck in the barrel had it partially fired.

Ah, gotcha.

I thought by the way it was described above that the round did not fire. Could have been half load, or poor propellant.