Blowing out happens because the extra expansion of the bread in the oven, called oven spring, expands further than the skin of the bread can accommodate.
The protein sheets that make up risen bread can toughen when they dry out. When the bread is introduced to the sudden high heat of the oven the water and alcohol vaporizes, filling the loaf with gas. If the outer gluten sheets were allowed to toughen or if the spring is just to great then the sheets have to tear to make room. The tear will happen wherever the sheets are weakest. This seems to be usually in the ugliest spot possible for a blowout.
It would be possible to reduce the spring by baking at a lower temperature but the spring is desirable and results in a lighter texture. The solution must come, therefore, in softening the gluten or controlling the expansion.
Fat softens bread so this problem happens mostly with lean doughs. One possible solution for some breads is to increase the amount of fat in the dough.
Keeping the surface of the dough moist will also help. Keep the dough covered while it is proofing and spraying it with water when it goes in the oven, and, if possible, having a humid environment in the oven will keep the proteins from drying out.
The traditional solution with lean doughs is to allow the blowout to occur but control the direction. In this way the loaf can be both light and attractive.
Slashing the surface of the loaf releases some of the pressure and allows the baker to have some control over the direction of expansion.