I notice this more on tins coming from C&D. I assume that it's for two reasons. One, they use the nice pop top tins which probably swell more easily that coin twist. Two, McClelland, the other major blending house using pop top tins tins their tobacco blends in an environment that is oxygen depleted (or so I have heard) so the higher oxygen amounts in the C&D tins simply promotes quicker and perhaps more obvious early onset reactions (aging). At least that's what I think, and I'm no scientist. Another thing I believe is it doesn't hurt anything. A good percentage of my cellar is pop top tins, most of the older tins are swollen and none of these swollen tins that I have opened in the past 15 yrs or so were anytthing but perfectly aged to the extent that time had permitted. If anything, the swelling is your assurance that the tin's seal and seams are intact.
Mike S.