It is a sidewall bubble. It is a bulge protruding from the sidewall of the tire. It is caused by air leaking from the inside of the tire into the carcass or body of the tire. The vast majority of bubbles are caused by impact damage.

That's just acne, should clear up in a few days..

Driving through Magma?

I have no idea unless the tire is faulty. One sidewall bulge from scraping a curb or something I could see. Unless this person routinely rams the curb parallel parking

One way to think about it is like a hernia in a human. Where there is muscle weakness or strain, the muscle separates and can allow the intestines to slip into the opening, which becomes a life-threatening emergency. Same concept, except with a tire wall and air instead of an abdominal wall and muscle/intestine.

Old age and multiple/extreme heat cycles is the most common culprit.

as PupGirl says - internal wiring in the tire lost integrity, and pressure made a bulge. That's a road traffic incident waiting to happen, shortly. Very shortly.

Every time I air up any of my tires, I always weirdly worry that they will blow up. Always.

It's most likely a soft sidewall where as sonebody drove on it while it was low. When you drive on a low tire it eats the sidewall and makes it super soft from the heat and lack of psi ... Or it could very well be a defective tire I've seen worse come into the shop and take for instance this tire in the photo is when the costumer says to me... Can you possibly fix this? Umm sure by taking it off the rim and putting on a new tire... Some folks just simply don't understand how dangerous the situation can be driving on tires like this..

So what did the driver do to cause that - body slam the tire a dozen times?

It's the corona virus!!

Exactly what I was thinking: How in the heck does that even happen?!

this is so weird.

Aliens

Smallpox!?

Zits

Good question! Yikes!

Driving into the gutter while parking ?

Tire herpes. This is what happen when rolling around without protection.

I personally don't know ;-;

Tire overfill

mumps

Riding on a under inflated or Flat tire, it causes the tire to rub and wear inside all the way around. Then when reinflated, the air pressure of the tire then escapes through the damaged inside of the tire and past the belts where it bulges out on the outside rubber of the tire wall. (if you look closely at the image you can see a little ridge or raised area running between the bulges). OR another scenario would be that the tire was hit so hard (like a massive pothole) that it caused the steal and nylon belts to separate inside of the tire.

I'm guessing wildfire.

Or the mumps on steroids

What happens when they pop?