Leakage Currents

Simple answer, because of capacitance.

http://en.wikipedia.org/wiki/Leakage_(electronics)

http://en.wikipedia.org/wiki/Capacitance

Additionally try a google search for more details.

The above answer is best ignored.

How long piece of string?

Leakage is defined as "unwanted current". In an insulator or a semiconductor it can be caused by thermal activation of carriers. Or it can be caused by impurities that provide hopping sites for electrons or by ionic conduction or...

Or there could be traces of impure liquid between the "isolated" wires - leakage here is often (but definitely not universal) due to ionic conduction.

Wiser? I feared as much

In AC mains systems, current passing through the capacitance between the phases (often most significantly between "live" and "ground") is also referred to as leakage. That is quite fair, as it is usually an unwanted current. One difference between this and those described above is that this is systemic; that leads to differences in terminology - in an electronic circuit this would usually be described as "stray" current" rather than as leakage.