The dock connectors are prone to damage as the USB cable is being plugged in and disconnected several times a day (due to the iPhones poor battery life!). With speaker docks its easy to accidently bend the phone the wrong way causing damage to the connector inside the phone. It only takes one yank at the wrong angle to break the connector inside. Sometimes the connector can be damaged due to physical damage, also we often see corrosion where the connector has got damp or wet, for example in a coat pocket.
The repair of the dock connector is relatively straight forward. It involves removing the old connector from the board and replacement with a quality spare part. Once repaired the new connector should have a long life assuming that similar damage doesnt occur again. The solution here is to be careful when plugging in the cable, or dock.