Heat is the prime killer of HID ballasts.
WE do a lot of parking lot/sports lighting, and have ballast failures from <6 months to >15 years. Voltage spikes can hurt too, if they are severe and occur often.. the 120 primary winding is the "weakest" link spike wise.
The "other" theory is '96 to '02 is 6 years, the mfg warranty probably was five. You can e-mail Advance/Universal for input also.
Your ionquiry as to normally "short out", it's a 50/50 split...but when the 480 one's short, and the "maintenance" guy resets the CB a "few" times....now that's fireworks..