Do you mean that it just appears at the right, or that you actually see it scrolling to the right as if you were clicking the scroll arrow or using a scroll wheel on your mouse?
If the latter, I've had a similar problem with my USB gamepad. For some reason it sometimes acts like the directional button is pressed even when it's not. After the first time I actually press it, it usually fixes itself (for a while, anyway). I'm not sure if this is a problem with my gamepad or USB Overdrive, which I use to drive it. In any case, if you have your gampad's directional buttons set to scroll by default, that could be the problem. It might also be some other kind of input device.
Do you use USB Overdrive? Or any kind of driver that can control scroll wheels?