Well, the cake carrier is partly to blame. Leaving it closed up in there can cause it to "sweat" alittle. At the very least it can keep it from getting cool air circulation. Fondant and gum paste are very picky when it comes to heat and moisture. Leaving the cake inside a plastic container, can contribute to both happening. Especially this time of the year, and depending on what area of the country you are in.
I am in the Pacific Northwest, we leave nothing like this out of the fridge, or in a place where it is likely to be negatively affected. Especially if it's going out to a client. If the heat doesn't get it, the humidity will. :/
From looking at your cake, and it's lovely by the way, you got a little of both. Sad, but a worthwhile lesson! Now you know!
Caking is a lesson of trial and error and eventually success!