Random fluctuations in the AC line and spurious electrical impulses generated by other devices including refrigerators, fluorescent lights, laser printers, etc., all contribute to this phenomenon. Unfortunately, it's not just noise. These emissions can affect the operation of certain types of equipment and cause data errors or even damage equipment. Most brown-outs occur when demand is too high and the power grid can't keep up. The flip side of a sag is a surge.
Surges can occur after a sag and damage unprotected equipment.
Many newer UPS devices provide serial connections to computers that, when equipped with the appropriate software, will notify users of an outage, close system files and then turn the system off.