Frequently you will see a brownout during a heat wave when more people than normal have air conditioners on full. Maybe you'll get yours just as that important file is being written to disk. Temporary voltage increases are fairly common. Large motors or circuit breakers in industry can put them on the electrical line. An appliance in your home can cause a spike, particularly with older wiring.
Lightning can put large spikes on power lines.
And, the list goes on. Electronic components are stressed over time as they heat up and cool down. Mechanical components simply wear out. Some, however, can be slow and not obvious. You can have hardware problems on a perfectly healthy PC if you have devices installed that do not properly share interrupts.