Tire casing appears damaged
Age is the most common cause of this sort of failure -- the tire cord simply breaks down.  It's worse in white-wall or "skin-wall" tires where black rubber does not cover the sidewall.  This allows sunlight, ozone, and any petroleum fumes in the air to attack the gum rubber holding the sidewall cords together.

And, of course, under-inflation causes excessive sidewall flex and subsequent cord breakdown.  Combine it with age and the tire gets a double whammy.

Beyond those, it's possible to "bruise" the tire various ways.  Hitting curbs, eg, especially with under-inflated tires, can create a bruise in the sidewall (in addition to the danger of "snake-bite" tube damage).  Or the tire may damaged in storage when a garden implement is pressed up against it or some such.