The material was mechanically removed from the side of your tire. It wasn't a result of any out of balance, heavy braking, skidding, overinflation or any other tire fitment reason. For example, a shred of shrink wrap with a little chunk of pallet, protruding staple points, snagged in the plastic, could spend time bouncing around on the junction of tire and swingarm, chewing that edge down. Riding in adverse conditions, like the rain, would make it hard to detect an insidious incident like that.
The fact is, if you tried to manually remove that much material from the tire, it would take a belt/disk sander, razor knife and at least 15-20 minutes of honest work at the bench, to accomplish that kind of damage.
Chewing up a specific edge of the tire, like that, takes outside influence.
Another example is when my muffler wasn't properly spaced away from the new wider rear tire. The muffler clamp wore down the edge of the tire, each time the suspension compressed far enough for contact.
"Big Bertha" my KTM 950 SliderMoto with 19/17 spokes, FCR41's, 2-into-1 high pipe, single side front brake and handcut flattrack style tires. Gettin Slideways is good!