Comment 35 for bug 1585084

The volume notification pops up when the system *thinks* the volume is changing, or the audio device is changing (per PulseAudio config or just something un/re-plugged).

I'm not sure, but it might also happen with some earphones/headphones if your system is trying to interpret volume buttons. If the socket supports them but is a bad fit for the particular headphones you have then you might get repeated spurious volume popups.

So generally if you experience this bug you need to do some detective work and try to find out what of those your system incorrectly thinks is occurring.