Why do you become catabolic when you are carrying bodyfat? There could be many explanations, likely, I'd imagine, is that when you are carrying bodyfat you want to lose it and begin a diet strategy.
In the face of a reduced energy intake, reduced carbohydrate consumption, increased activity levels and reduced processed foods, glycogen depletion and fluid losses are quite easy to achieve, resulting in the apparent loss of lean tissue.
Obviously this will depend on the extent of the above factors, but in reality true muscle loss - as in the loss of contractile proteins - as long as an individual still trains at a high intensity and manintains a suitable protein intake - is a lot smaller during a diet than you would imagine. Intense resistanct exercise can, and studies do suggest this too, help preserve a significant amount of muscle tissue (some even suggest that lean mass could be gained, however, this is both rare and pretty unlikely).
The comments by BBB hold true too. From an evolutionary perspective bodyfat is a pretty important organ (some 'authorties' view fat tissue as an organ and not just stored energy), which may help regulate appetite, metabolism and satiety.