What causes a transmission line to be unbalanced?

In an unbalanced transmission line, such as a coaxial cable, one conductor (the outer shield) is connected to ground. The other conductor (the center wire) carries the RF signal relative to that ground reference. In contrast, a balanced line uses two identical conductors that carry equal and opposite currents, with neither tied to ground. Distractors like poor connections or deteriorated insulation describe physical faults, not the fundamental electrical design of an unbalanced line.