Interesting failure. In other things that would usually be wear or corrosion in the mid point of the volume wheel (or there being some kind of hard lock/bypass at the extremes, think with a clunking lock you might have seen in some devices, but the GBA is not sporting such things) but I don't think I have seen it on a GBA before, though it is now long enough that I suppose we are going to start seeing such things and the GBA is the same as any other audio sporting device with a similar setup as far as what it could be prone to.
Some might get around it by getting some contact cleaner in there and running the wheel back and forth, a few mad types might even disassemble the volume wheel and clean/fix it, most would find a replacement and there does appear to be a few around. Fairly basic solder job as well.