By overuse. Apparently musicians may get it too, so I am told. I worked in a maple syrup factory when I was younger. Filling up those leaf shaped bottles, I had to move it side to side to get the bubbles out for proper filling. It doesn't seem like much but its always a flick of the wrist 4-5 times a bottle. And gotta work fast cause its boiling syrup filling the bottles. We would fill over 100 bottles easy in a day.