Cigars burn unevenly for several reasons. If you do not light a cigar properly, it may burn unevenly. It’s very important to “toast” a cigar before you light it. This will insure an even burn.
Another reason for an uneven burn is the humidity of the cigar. If part of the cigar is dry and part moist, it will burn unevenly.
A windy day can also cause a cigar to burn unevenly. The wind will accelerate the burn area where the wind hits it.
Cigar construction can also affect the burn. If the filler is not bunched correctly, the cigar may burn unevenly.
A plugged cigar will cause an uneven burn because the draw will be uneven.