There are dramatic things that cause power outages: exploding substations, ice storms that cripple the entire Eastern seaboard, and heat waves that engulf the country and place enormous air conditioner-driven strain on the power grid. Then, there are the less dramatic sources like, well, squirrels.
Although customer-for-customer, a flood or a massive storm will typically affect the highest number of people, in terms of actual power disruption events (whatever the scale), no storm can hold a candle to the antics of squirrels.
The cute and furry, but horribly destructive little creatures love to use power lines as their personal cross-country highways. The lines are high, convenient, and offer safe routes over roads near food-bearing trees and roof-lines: all places that squirrels love to go. This wouldn’t be a problem of course, except that squirrels are long enough that their bodies can easily bridge the space between wires, transformer components, and other pieces of the electrical grid that can lead to a short circuit.
Under ideal circumstances, the electrical shock knocks the squirrel’s body free from the equipment (and the flow of electricity resumes), but in many instances, the body of the squirrel remains lodged in the equipment, triggering a continuous fault that burns out circuit breakers and nearby equipment. The squirrel problem is significant enough that many utility companies have resorted to installing squirrel deterrents, including the kind typically seen on bird feeders (rotating baffles, bushing to cover sensitive points on the pole and transformer, and even using predator urine to discourage squirrels from climbing on particularly hard hit poles and substations).