How did this happen?
The reason this problem exists is because of a large-scale industrial espionage foul-up. Some companies decided to steal an electrolyte formula from another competitor. Little be known to them, the stolen formula was incomplete and flawed. They didn't discover this until it was too late and they had manufactured and distributed literally MILLIONS of these flawed capacitors. It was way too late for any kind of recall, and even today, these crappy components are being used in new boards. As I mentioned before, I believe this problem runs much deeper than simply an industrial espionage screw-up, as that incident was exposed years ago, and the problem still exists today. Nowadays, it just boils down to corporate bean counters cutting corners to save money by using shoddy components.
This inferior and flawed electrolyte formula was used by a number of component manufacturers that sold to many different, reputable, and well known motherboard manufacturers. This problem isn't isolated to one particular brand of motherboard, and not even isolated to motherboards alone. I won't mention brands, but a VERY popular monitor manufacturer has been plagued with RMA's on some of their monitors that were built using these inferior capacitors. This problem has been reported in computer motherboards, monitors, televisions, radios, and stereo equipment. Through my experiences owning a service center, I've personally seen and serviced a large number of 'high-end' equipment that had prematurely failed capacitors.
More detailed information...
In more technical terms, this is is what actually happens to the capacitor...in the simplest of terms. Think of an electrolytic capacitors as a battery. They are designed to store a charge and release that charge depending on the specific requirements of the circuit. Inside the capacitor there are two metal plates with dielectric material between them, wrapped in paper, filled with acid (electrolyte), and sealed in its housing or 'canister'. What happens is the flawed electrolyte prematurely deteriorates and dries up. When this happens the capacitance value changes, becomes erratic, and can even short completely, which obviously causes the circuit to malfunction. On your motherboard, this results in system instabilities or complete failure of your board.
From a physical standpoint, the capacitor can display a number of symptoms and even have catastrophic failures. Catastrophic failure is a rare phenomenon, but it does happen. The reason is this... A capacitor canister is completely sealed and air tight. When the electrolyte dries it turns from a liquid state into a gas. This gas expands with heat and builds great pressure inside the canister, the theory is the same as a pressure cooker. Of course the obvious happens when that pressure builds too much, and the capacitor will vent. This is what causes the capacitor to 'bulge' or swell up. In a catastrophic failure, the capacitor may actually burst or explode. It can sound like a firecracker going off or sound similar to air escaping from a car tire, depending on how high the pressure has built. A physically failing capacitor has an ammonia-like odor (at least that what I thought it smelled like)... Capacitors that has swollen up are easy to detect, but one that has burst are even more noticeable. It will usually blow from the top and spew its innards throughout your computer case. The innards are mostly paper and any remaining electrolyte, but it sure can leave a mess... Once again, catastrophic failure is a rare phenomenon... It's not dangerous either, as long as you practice common sense safety precautions! If you suspect bad caps, don't put your face near them while the system is powered up, and don't eat the paper or electrolyte that they blow out!