What caused it? Flash trigger voltage that was many times more powerful than the DSLR's circuitry could handle over time. Trigger voltage is stored in a flash's capacitor, and after being put through a step-up transformer, it ignites the xenon gas used in virtually all flash tubes. In many older hot-shoe and studio strobes, that voltage can, unfortunately, ricochet back through the flash foot (or PC terminal) into the camera. The most popular flash unit of all time, the Vivitar 283, for example, used a 20-volt flash trigger -- more than three times the amount some DSLRs were designed to accept.