What's with all the moldy beer?

Moldy beer bottles, that is.  I have been to 3 separate grocery stores recently where I have found mold growing on the outside of the beer bottles.  Somehow it's always been the beer I wanted to purchase!  Some of the beers I passed up, others I cleaned off before or after buying.  Perhaps it's my own fault for buying beer at a grocery store instead of a proper beer or liquor store.  Whatever the case, I would just like to know where this mold is coming from or what is causing it!  Can anyone tell me?