Every reenactor will tell you how this could happen. In the noise of battle unless you are shooting indivdiually, independent fire or skirmishing, you most time will not know that your gun has fired. You simply know that you pulled the trigger and you guns report is lossed in the multitude of other weapons being fire around you while you are in formation.
All it takes is one misfire to start the chain of loading multiple cartridges. Those soldiers were lucky that the first misfire did not discharge on the second, third, fourth, fifth, or more reloads. Those "trained" soldiers surely didn't drop every cap every time they reloaded. Even in Artillery in reenacting, you are taught to watch the muzzle of you piece to make sure that it discharged. And to worm between each shot primarily to clear the tube of hot embers, but also to physically check that the charge has gone off.
You simply can not depend on noise alone to tell you that your weapon has discharged. And you would think that standing beside a cannon going off would be a loud enough noise to tell you, but it isn't.