Why does my fluorescent ceiling fixture trip the GFCI sometimes?
Interference
The problem could be caused by Electromagnetic interference, or an attempt to filter the interference.
Older Ground-fault circuit interrupting (GFCI) devices may be more susceptible to EMI related nuisance tripping. If you have an older GFCI device, you should first try replacing the GFCI device.
Some fluorescent fixtures have an EMI/RFI filter built into the device, to try and prevent any interference produced by the fixture (ballast or bulbs) from leaving the fixture. In most cases the interference is bled off through the equipment grounding conductor (EGC), which can lead to an imbalance of current between the ungrounded (hot) conductor and the grounded (neutral). This current imbalance may be large enough, and long enough to trip a GFCI device.
Ground-fault
It's quite possible that there may actually be a ground-fault somewhere in the fixture, or the switch. Could be a stray arc somewhere in the fixture when it turns on/off, that causes a small amount of current to leak. It might be worth trying a more modern fixture.
Why is it GFCI protected?
You may want to figure out why the fixture is GFCI protected in the first place. If the protection is not required, removing the GFCI protection will certainly solve your problem.