The casing of my back tire appears to be damaged. There is one spot where it is torn, and the tire bulges out when the it is inflated. I want to know what causes this so that I don't do the same thing to my new tires. Also, if there is a way to fix this, I'd like to know how.
Age is the most common cause of this sort of failure -- the tire cord simply breaks down. It's worse in white-wall or "skin-wall" tires where black rubber does not cover the sidewall. This allows sunlight, ozone, and any petroleum fumes in the air to attack the gum rubber holding the sidewall cords together.
And, of course, under-inflation causes excessive sidewall flex and subsequent cord breakdown. Combine it with age and the tire gets a double whammy.
Beyond those, it's possible to "bruise" the tire various ways. Hitting curbs, eg, especially with under-inflated tires, can create a bruise in the sidewall (in addition to the danger of "snake-bite" tube damage). Or the tire may damaged in storage when a garden implement is pressed up against it or some such.
It can be caused by a number of things. Something catastrophic can occur, like taking a corner on an under inflated tire and rolling the tire off the rim and sliding down the road on the side of the tire, or something small which then gets worse over time like a pinch flat or even a manufacturing defect.
In the event of the small issue (before the tube bulges out) you can prevent the problem from worsening by sewing it up with dental floss (I prefer cinnamon). I've had this technique hold on some Gatorskins for a few months, but at >100 PSI it's only a matter of time before some tube starts to show and then bulge.
The most popular solution is to use a boot. People like to use dollar bills as boots. Just lay the bill inside the tire between the tube and tire. When you inflate you should get no bulge.