Why do power lines sag more on hot days?
The Real Answer
Power lines can sag more on hot days because the metal conductors expand as they get hotter. The supports stay roughly the same distance apart, so when the conductor becomes slightly longer, the extra length shows up as a deeper hanging curve between those supports.
Ambient air temperature is only part of the heating. Electrical current flowing through a conductor also produces heat, so line loading can add to conductor temperature and sag. Wind and other operating conditions matter too, which is why utilities design and maintain clearances for more demanding conditions than a cool, still day.
Temperature-related sag is therefore a normal engineering effect of a suspended conductor changing length with temperature, not evidence that the poles themselves moved farther apart.
You notice one power line looks low and immediately start comparing it with the roofline, the tree, and your memory of Tuesday despite never having measured a power line in your life. Five seconds later you're standing on the sidewalk lining the wire up with the same tree branch three times, then you go inside having told absolutely nobody.
Why People Ask This
People ask because power lines look like fixed pieces of infrastructure, so a visible change in their curve can look like something loosened or shifted. The missing piece is that the conductor itself changes length as its temperature changes while the support points remain in place.