When a voltage passes through a transformer it has lag, not much but it does. You can take the light and check the continuity with an ohmmeter like I mentioned then plug it into a GFCI and just turn it on and off and I bet it will trip at least once from that.
This is speaking from experience that I have had on the job (as an electrician) and personal experience with my own tank setups.
35gal Reef, 20gal Sump--2 years, 3 months--Shut Down
If you can't fix it with a hammer, you have an electrical problem!