Why do physical movements of cables create hundreds of picoamps of current?

Why do physical movements of cables create hundreds of picoamps of current?

There are several ways that cables can generate error currents. First, the cable materials can generate currents similar to the way a piezo-electric crystal can generate charge. Second, friction between the concentric layers of a cable (coax) can generate static charge that flows along the conductive surfaces. These currents could be on the order of 100pA.