Nucence tripping? Possible that it trips on it's own with a lower load, but normally that is when something makes it do so such as phase
harmonics, larger lighting loads, voltage drops, shorts, someone kicking the
dimmer pack
etc. As Mayhem said, there could be any number of reasons a dimmer can trip or nucence trip. Try re-patching as he observed especially into a different phase of power, in a different breaker of a different dimmer pack and observe. Could be some dimmer trip ratio has warn some. I have some 100amp four pole main breakers that take a lot of effort just to turn on not that they can be trusted for a show with a similar problem. The thing about such a breaker that is warn is in turning it on and off. Said breakers that trip too easily will most likely not like to go on and off. They will instead go from off to tripped mode instead of on.
128% by the way is the normal temporary pre-tripped loading possible on a
circuit breaker I read somewhere once. Just about enugh time to melt down a 100amp main breaker when seriously un-balanced - my own education in balancing your load. This balancing your load and just an idea, how is the loading of the dimmer arranged? Is your load balanced and where is this breaker placed on that power distribution? Might if daisy chained and feeding a heavy load be possible that the extra resistance/current even passing thru the breaker's in terminal if that type, could be sufficient to overheat the breaker especially if first to the point it trips at a lesser loading.
Than again it could also be electronics and lots of other things. Follow the process of elimination first. Narrow it down first to if it's the circuit fed or the dimmer feeding. Given the dimmer feeding, is it the elctronics, wiring to it or the circuit breaker itself? If possible swap out dimmer cards, and even circuit breakers (supervision required) to narrow down the actual cause. Once you track it to the breaker, it's not un-common that a circuit breaker especially if used as a switch which it is not, will wear out.