often when that happens to me, I can tell it was because of the flaw in the way the cigar was rolled. I think it is called "booking"? Look at the end of the cigar, the tobacco should be perfectly rolled in a circle or curve, but sometimes you see where it isn't, and instead of curves you see straight lines, and that can cause an uneven burn. Sometimes it will eventually correct itself. I know I am not doing a very good job of explaining this. Also the problem is often associated with a problem with the actual construction of the leaf.