Tire age
A fellow car club member had this happen on the interstate last year, in the left lane. Tread separated from the tire. Fortunately he was able to pull over without incident but the carnage happened in the blink of an eye.
I know mine are well past their safe age (but no visible dry rot) & I think about it every time I'm driving.
Trying to hold off until my front disc brake conversion & rear axle swap with LBP 15" wheels (hopefully by next year).