You have capacitance between any 2 insulated conductors in a raceway or cable, as well as between the conductors and ground. A capacitor is simply 2 conductors (the copper wires or metal raceway/armor) separated by an insulating dielectric (the wire insulation). The potential leakage gets higher with a longer cable run (adds more capacitance) or older, degraded insulation (lowers leakage resistance).
If the resistance gets low enough or the capacitance gets high enough, sufficient leakage current can be present to trip the GFCI, or to lower the "overhead", so additional leakage below the usual 5 mA will be enough to cause a trip.
[This message has been edited by NJwirenut (edited 03-27-2005).]