Volume on Mac increases automatically when earphones plugged in

I have a MacBook Pro running El-Capitan v10.11.5. Whenever I plug in my earphones, the volume gradually increases (one bar every couple of minutes), automatically. This problem has been going on for at least some months, fairly sure this happened also in earlier OS versions.

I didn't find anything about that online, is that a familiar issue? How can I solve it?

Sounds like you have headphones with volume controls and either they volume up bottom is jammed/broken, or they're not compatible with macs... The first one is my guess. The second one is rare.

I too had the same issue on my Early 2015 MacBook Air when paired with Samsung Earphones with volume controls. Unplugging the earphones and plugging it again resolved the issue for some time. Try rotating the AUX plug, to a different position. It worked for me.