The early, unexpected, but welcome retirement

My vehicle recently developed a horrible shimmy that had my hair turning gray.  The thought of bringing it in to a mechanic, only to find out I needed to shell out something like $1200 was horrible.  The first thing I did was take off the tires, one by one to make sure there was no damage to the wheels & to clean the insides off.  I scraped a great deal of mud off of the rear tires, and hoped that this was the cause.  Past experience with dirty wheels has shown me that this may cause a horrible imbalance and making the car wobble a bit.  Seeing there was no damage, I raced off in hopes that clean wheels would solve the problem.  It didn't.

Not sure what to do, I tried to ignore the problem for a while.  I had bigger fish to fry, so the shimmy would have to wait.  Each time I slammed my driver door and took off, the shimmy only got worse.  Hard to ignore.  But first, I had to go buy some vegetables to go with dinner.  On my way back to my vehicle, I noticed that the left rear tire looked horribly bulged out in one spot.  A closer look showed that it was in fact bulged out. 

When I got home, I immediately changed that tire and sped off to see if my problem was fixed.  It was!  I have to wonder why the tire failed like so.  There were no visible signs of damage to the exterior of the tire.  Something on the inside gave way, I suppose.  But what, and why?  The tire was not old at all.  It still had years of use left in it, if not for this failure.  The possibilities on why it failed are endless. 

My guess is that a defect in the rubber on one spot of the tire waited for the sunshine to hit it, while driving 45 mph.  While at the same time, the tread near that defective spot hit a golf ball sized rock, and then on the next revolution- a little puddle of water was the last straw, because it caused a chemical reaction making the rubber stretch a little.  Just lucky I found out what it was, before a blow out occurred.