Wear and Tear, Constantly plugging in and unplugiing, having it plugged in and stress on the cable, pressure on top of connector while plugged in, and the all time favorite, on a desk..plugged in and you or someone comes and trips over the cable, adding stress. Some of those DC jacks on the laptop motherboard are just soldered poorly, repeated usage and they tend to get lose, the pin breaking thru the solder. Some people have the laptop in bed, sitting back with the laptops placed in legs while they in 90degree'ish arch with the dc jack cable being pressured against their front legs.