Since the processor when heated generates more heat than when it was at temperature when its temperature reaches 90 degrees Celsius it begins to reduce the speed of its rotation under the order of processor thermal protection system. The fan is enabled again only when the temperature falls below 80 C.
If you’re experiencing spikes in your computer’s CPU usage all day long that never stop, then either your keyboard or mouse is hung up and doing work that isn’t stopped, so unplug them to see what happens. There are also rogue programs running on your system which consume resources even if they’re not needed anymore – these will result in high load for hours or days at a time until you notice them and kill them manually.