The tube may have suffered arc over and you saw the results of that;but I also think Elevick's suggestion is one to also consider.
Tube Fireworks
Tonight I turned on my amplifier, starting hearing popping and hissing from one speaker, and turned around to find one tube sparkling like fireworks! Obviously the tube is bad and I won't use the amp again until I can replace the tube, but does anyone know why such a thing would happen? Thanks!