[quote=SafetyWiredMy thinking is that the lighting circuit has no, or a bad earth ground and caused the trip.[/quote]
Actually it sounds like all of the bonding paths and grounded conductors are working correctly.
What you have is clearly a case of poor protective device coordination, as was previously mentioned.
Most manufacturers will ship their GF devices set to the absolute minimum. If these settings are not changed the main GF can very easily trip before a 20A branch device operates.
I would guess that fixture and ballast changes result in the predominant number of main GF nuisance trips.
