Alright, so after a few more hours, I've had my keyboard AND my USB shut off along with the mouse, but it only happened when I was quickly moving my mouse around.
Moving my other mouse around, however, and everything seems to be fine. It seems that moving around the faulty mouse was the only time I got my other USB devices to shut off.
Is this still the mouse's fault? Or the mobo? Mobo still seems unlikely though because, as I've said, using the other mouse doesn't lead me to my other USB devices shutting off (nor does that mouse even turn off).