It could be a wide range of things... Guitars are made out of wood and wood is pretty absorbant. Sometimes moisture gets into the seams where the frets are and makes the wood swell in those areas... pushing the fret up... causing fret buzz... Or it could be that you have worn that specific area of the fret down to where it buzzes.... High frets and low frets are very common. Usually a way to counteract a low fret is to higher the point of origin of the string.... Point of origin being the Bridge... If your guitar has bridge saddles you can adjust just that one bridge saddle... it doesn't take much to get rid of a buzz... usually just raise it a millimeter or so. This gives the string extra clearance down the neck. Don't bother adjusting your truss rod... it's not worth fiddling with that if the buzz only affects the one string. You'll just end up with a guitar that doesn't play as well as it should... all because of one string. Try raising the bridge saddle.... If your guitar has a stop tailpiece (Les Pual style)... Raise the side that is closest to which string you're getting a buzz.