Weather is the leading cause. High wind brings tree limbs down onto distribution lines, ice and heavy snow load and snap them, lightning strikes equipment, and hurricanes do all of these at once across a wide area. This is why outage maps light up before and during major storms and why restoration after a hurricane can take days.
Extreme heat causes outages from the other direction — through demand. When millions of air conditioners run at once, electricity use can approach the limit of what the grid can supply, and the strain can overheat and trip equipment. Heat waves are now a leading driver of summer outages.
Plenty of outages have nothing to do with weather. Equipment simply wears out and fails; a vehicle hits a utility pole; squirrels, birds and other animals bridge energized parts and cause a short. These tend to be local and short-lived, the scattered single-street outages you see on a calm day.
Some outages are deliberate. During extreme fire weather a utility may run a Public Safety Power Shutoff (PSPS), cutting power preemptively so its lines cannot spark a wildfire. And when demand threatens to outrun supply, grid operators may order rolling blackouts — also called load shedding — rotating short, controlled outages across neighborhoods to keep the whole system from collapsing into a far larger, uncontrolled failure.