About 2 years ago, I was driving my 58 Chevy, and there was a HELL of a bang that shook the whole car. Couldn't figure out what it was until I opend the trunk. Yup, my spare looked just like yours (fortunatly, no damage to the car). The tire had been pretty used up and the amount of tread looked to be about the same as you have, that had not bothered me too much as it was only going to be a spare. Thew tire had been in the trunk for about 12 years, and except for the ocassional checking tire pressure (32-35 lbs) had just sat there until it exploded.

Besides being an old tire, I figured that part of it was the car sitting out in the sun baking that day and the temp rising in the trunk.