The only way that a tube gets a 6" gash in it is if part of it is at least momentarily outside the tire so this part of the tube blows up like a balloon and bursts (this is not the fault of the tube). Once it deflates the tire frequently reseats itself with the tube inside so it isn't always obvious what happened. As 10wheels said above, one common cause is if the tube was pinched between the tire and the rim. When inflated the pinched part of the tube works its way out between the tire and rim and then explodes. Another possibility is a bad spot on the bead of the tire that doesn't seat properly against the hooked part of the rim edge. When inflated the tire pulls away from the rim at this point and lets part of the tube escape and blow up. A damaged edge of the rim can also cause this but is less likely.