Why is FAD, rather than NAD+, reduced in the succinate dehydrogenase reaction of the Tricarboxylic Acid Cycle?
In general, $\ce{NADH}$ and $\ce{FADH2}$ are coenzymes. The structure of the main part of an enzyme determines which coenzyme or which prosthetic group will work with the concerned enzyme. Unlike most other TCA cycle enzymes, Succinic Dehydrogenase involves the participation of $\ce{FAD}$ rather than $\ce{NAD}$ and that is a consequence of its specific structure.  

Another possible reason might have to do with the energy gap available. The conversion of succinate to fumarate liberates less free energy as compared to other oxidation reactions. Ususally, the steps which are coupled with NADH reduction have a free energy change of about $\Delta G=-100$ to $-150~ \pu{kcal/mol}$ but the conversion catalysed by succinic dehydrogenase has a free energy change of $ -80~ \pu{kcal/mol}$ (rough figures) and hence releases less energy. It therefore might not be feasible to couple with $\ce{NADH}$ reduction but it would be favourable (thermodynamically) to couple with an easy-to-reduce (since the reduction potential is higher and hence reduction is more favourable) and hence less energetic molecule $\ce{FADH2}$.  

Apart from structural and thermodynamic requirement, one last possible reason might be that succinic dehydrogenase is a part of the mitochondrial membrane and involved in the Electron Transport Chain. Here, flavin molecules are excellent electron transporters (in comparison to nicotinamide based reductants) and therefore it might be more profitable to use a flavin based reductant to later facilitate easy electron transport and proton transfer.
Again all these are probable reasons and I dont know for sure.