Listen to what your tires are telling you.

Good catch

 

What brand of tire and what position? Just curious.

 

Impact break. Most likely from past curbing. Rust is from moisture in inflation air getting to steel ply

 

Could it be from potholes, some can’t be dodged? Or from before it was mounted, maybe being tossed around, or dropped from high up?

 

Pot holes and other hard hitting road hazards can do that easily. The only thing that could have done that prior to mounting was if it was stabbed with a forklift or pinched with something heavy before or during shipping which is highly unlikely. The only way it coulda happened during mounting is if the bead was torn. Which happens a lot more with dismounting for repairs or to move to another position. Since the bead was not torn. “Road hazard” Impact with something is most likely the cause. But in any case this is prime example of why an inflation cage should always be used even when tire is brand spanking new.