My theory: it's a design flaw in the way that volume + track control work through the headphone jack.
My S5 came with a set of headphones that have an integrated control for volume + track control, and I do not seem to have any issues with volume adjusting itself when I use those headphones. BUT, if I have my phone connected through my car audio AUX in or I use third-party headphones, I do experience this problem. My guess is that the controls in the Samsung headphones issue commands to the phone via an extra ground connection in the jack.
The theory is that when using headphones / an AUX line that does not have built-in controls, the phone is misinterpreting small variations in voltage as commands to adjust volume. If you think about it, the variations would be caused by the phone's own power-saving, peaks in the audio for the track being played, etc.
So, basically, my theory is that: 1) it only happens with headphones that do not have integrated controls, 2) that the phone is too sensitive to control signals via the grounding pin on the headphone jack, and 3) small variations in voltage via the grounding pin are being interpreted by the phone as "volume down" commands.