I'm pretty sure that this effect is, in fact, caused by sparkles.
OK, let's unpack that a bit. A glittery surface sparkles because it consists of lots of little reflective facets (or pieces of transparent refractive material) that each reflect incoming light (e.g. from the sun) in a random direction:
These reflected light rays form a large number of very narrow beams, each reflecting off a single facet. When one of these beams happens to hit your eye (or the sensor of your camera), it appears as a small bright spot — a sparkle.
However, when you shoot a picture that's out of focus, the light rays that hit different parts of your lens aperture will fall on different parts of the sensor. Thus, it sometimes happens that the narrow beam of light reflecting off a facet hits only part of the lens, and thus illuminates only part of the resulting bokeh circle:
(Note: The diagram above is actually subtly wrong, but it illustrates the principle. As an exercise, you may want to try drawing the light rays more accurately, preferably starting from the focus and including an actual light source of limited angular size, to see what's really going on. Then think about why this "fake" diagram, which is essentially drawn starting from the reflected beam, predicts the correct result, even though at least one of the refraction angles is wrong.)
Basically, the effect that creates these uneven bokeh circles is the same as what causes the individual sparkles on a glittery surface to appear and disappear as you move your head — the beams of light forming the sparkles are narrow, so they can only be seen from certain vantage points. Your lens aperture is wide enough that, sometimes, only part of your lens "sees" the sparkle, while the other parts just see a dull facet that reflects nothing particularly bright.