yssy.ymel, to speculate for just a minute - I would guess you hit it right on there.
Being a SWA -300, I can only imagine that it is a high cycle airframe. Despite the best safeguards in place, fatigue related failure can occur when least expected.
As stated, the uniform shaped hole suggests that the tear strips did their job.
Now what would have happened if a pax/hostie had been stood under the hole a la the Aloha incident....