I've seen plenty C&D blends bulging but it's not only limited to them. Gasses will be a byproduct of fermentation no matter what content of the tobacco IIRC. Back to C&D, my friend got a tin of Comfort (in the Serenity Series) and when he opened it, the wrapper partially shot out of the tin along with some tobacco. He actually happened to be filming it and put it on youtube.