Why is this light fixture tripping the GFCI?
From your diagram it looks like the grounded (neutral) conductor connected to the light (that trips the GFCI), does not come from the GFCI device.  It looks like the grounded (neutral) wire is coming from the feeder to the circuit, instead.  

Because of this, you'll have current flow through the GFCI device on the ungrounded (hot) conductor that does not flow back through it on the grounded (neutral) conductor. The GFCI sees this as a ground-fault, since the current on the ungrounded (hot) and grounded (neutral) conductors are different.

To remedy the situation, you can either not provide GFCI protection to the light, or connect the grounded (neutral) conductor from the light to the LOAD side grounded terminal of the GFCI device.

Essentially, this is what it looks like you have now.

  

Notice that the grounded (neutral) conductor bypasses the GFCI device. 

No GFCI Protection

To fix this you could not GFCI protect the light, which would involve making a wiring change in the switch box. You'll have to move the wire feeding the switch from the LOAD side of the GFCI, to the ungrounded (hot) conductor feeding the box. The final circuit would look something like this.



In this situation, your original diagram would look like this.





GFCI Protection

The other option is to connect the grounded (neutral) conductor from the light to the GFCI, which would require running an additional conductor between the light box and the switch box. You'd then use the extra conductor to run from the grounded (neutral) LOAD terminal of the GFCI, to the grounded (neutral) terminal on the light.



If you go this route, your original diagram will look like this.





NOTES:


This answer is based on the assumption that your diagram is correct.
If local codes require the light to be GFCI protected, you'll have to do what is necessary to provide GFCI protection to the light.