Something leaks current. It could be a ballast shorting to
the fixture, a light socket shorting to the fixture, wiring
inside shorting to the fixture, or even wiring shorting
between neutral and ground inside a 3-prong plug that
is plugged into the gfci. The gfci itself could be shorted,
too (like between the ground and neutral connectors inside
the female socket, or an electrical equivalent of that on a
circuit board). Usually it is some device plugged into it rather
than the gfci itself, but they can fail.
(faqs)
http://www.faqs.org/faqs/electrical-...ection-24.html
http://www.thecircuitdetective.com/gfis.htm