Almost always tar, check the head, you see a spot that looks kinda dark and wet, cut the head back, problem will instantly resolve (assuming you cut enough to get rid of the build up). Usually from smoking to fast, but sometimes thems just the breaks. I've had it happen with various cigars, from some cheapie to AF Hemingway, so I wouldn't go making and rash decisions like putting the cigars in the compost or something.
There was a period of time when, since I had it happen a lot more during the summer, and I smoke outside , in Florida and thus put my trust in DEET, I thought it was cause I somehow got some DEET on the wrapper.