My earphones' cable has grown a pimple. It popped today.

Image on left is from 4 days ago, but the pimple was slowly forming over around 2 weeks.

The goop was sticky, not oily. Earphones are Panasonic RP-TCM130.

I was not able to find an explanation.
Something to increase cable lifespan, lubrication, rubber disintegrating, sweat and earwax that somehow got into the cable, dielectric grease, SCP-1407, no clear answer.

At first I thought the wires just somehow twisted. Nope.