The first time it did it to me, I know exactly what happened. I have a habit of adjusting the valve by squeezing the sides of the tire, then twisting the wheel in a circle, thus dragging the tire around slightly and straightening the valve.

But, my Gator was so tight on the rim, that it didn't budge the tire, and by the time I was done, I had frayed all the threads on my brand new tire.

On other tires, I think it was just age. A year or so of weathering, and the threads just went bad.

I took some photos, I'll look for them later.

As mentioned, my blowout was with a nearly new tire, so something else happened to it. Part of it was inflating it to full pressure during a cool evening, then taking it out for a long hot ride the next day giving me some overpressure, but I don't that was all.

Nonetheless, other than that one blowout, I've either gotten down to the threads (center of tire) with wear on my Continentals, or down to say 1/32 or so of an inch of tread left.

My theory is to some extent, thin tread is not too bad for riding as there is nothing left to hold onto debris.