Leakage somewhere?
The problem would have to either one of two things-
a) a surge during turn on (I'm assuming filament type low voltage lighting here, which would have a potentially large turn on surge if you don't account for this when sizing your breaker). It could be that the surge during switching on the lights is borderline, which would explain why it's only tripping sometimes. Need to look at how many lamps, total wattage, turn on surge per fixture verses running current.
b) Some form of intermittent leakage, possibly due to humidity or some other factor, is the cableing buried? Age of wiring?