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It's normal. They do this when they are disturbed. They rub the setae off their abdomen as a deterrent to anything that may be bothering it. A breeze, prey or even handling can cause them to do it. Also, right before they molt, they will lay a mat of this setae down on top of their webbing....I'm not sure if it's for extra "cushion" or what but I've witnessed it quite a few times, myself. The bald spot isn't dark, yet so it may not be too close to a molt. Time will tell. No need for alarm.
As Casey K pointed out it could be the T is reacting to something in its environment, so maybe try and think what could be causing it, then avoiding that thing.
Then again it could be part of the natural progression of the stages of moulting.
Then again, tarantulas can be goofy and you just don't know what or why they are doing that. I suspect mine are just trying to screw with my head.