This is likely because this is the area that you are holding your cue at when not at the table. Even tho you may not realize that you are sweating, you may be everso slightly. If your sweat contains a more acidic ph level, it will raise the grain slightly...enough that it is noticeable to you. Unfortunately, even if you get this area burnish out well, it will eventually do it again, simply because some of that acidity is still trapped within the pores...I had a shaft do the same thing, only at the top where I would hold it to chalk. My solution was to clean the shaft with Magic Eraser.
Here is a really great thread on how to clean and condition shafts:
http://forums.azbilliards.com/showthread.php?t=188985
The best way to avoid this problem in the future is to wash your hands religiously after every couple of games or so. It has worked out well for me.
Lisa
I think I'll give it a shot if one more (free) reconditioning by a local cue repairsman doesn't fix the problem. When I'm not shooting, my cue's either resting on a table or gripped very low, usually by the joint give or take a few fingers. I've got half a Magic Eraser in my case, but I don't want to risk bungling up the feeling of my shaft a week before a tournament. Related note, you just reminded me that my cue tip could use its last shaping... I've been using the same Sniper for the last year. Those tips are the nuts!