Power surges occur when something boosts the electrical charge at some point in the power lines. This causes an increase in the electrical potential energy, which can increase the current flowing to your wall outlet. Several different things can cause this to happen.
The most familiar source is probably lightning, though it's actually one of the least common causes. When lightning strikes near a power line, whether it's underground, in a building or running along poles, the electrical energy can boost electrical pressure by millions of volts. This causes an extremely large power surge that will overpower almost any surge protector. In a lightning storm, you should never rely on your surge protector to save your electronics. The best protection is to unplug them.
A more common cause of power surges is the operation of high-power electrical devices, such as elevators, air conditioners and refrigerators. These high-powered pieces of equipment require a lot of energy to turn on and off components like compressors and motors. This can create sudden, brief demands for power, which can upset the steady voltage flow in the electrical system.
While these surges are nowhere near the intensity of a lightning surge, they can be severe enough to damage components, immediately or gradually, and they occur regularly in most buildings' electrical systems.
Other sources of power surges include faulty wiring, problems with the utility company's equipment, and downed power lines. The system of transformers and lines that brings electricity from a power generator to the outlets in our homes or offices is extraordinarily complex. There are dozens of possible points of failure, and many potential errors that can cause an uneven power flow. In today's system of electricity distribution, power surges are an unavoidable occurrence.