The photosensitive nature of this compound is used in actinometry (look up ferrioxalate actinometry). Light reduces the Fe3+ to Fe2+ and the oxalate form CO2:
2Fe3+ + C2O42- (hv) --> 2Fe2+ + 2CO2
You can then add something like 1,10-phenanthroline if you want to quantify the Fe2+ and hence the amount of photons, by measuring the absorption of the iron-phenanthroline complex.
So in answer to your question, bonds do break as the complex goes via an excited state, and one of the pathways deactivating this excited state is decomposition.