Have there been any theories put forward as to the reasons behind Temperature-Dependant Sex Determination in reptiles?
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Environmental sex determination is tremendously interesting. 
It looks like the most common explanation is that temperature-dependent sex determination (TSD) is the original form of sex determination, and genetic sex determination has evolved multiple times in different animal groups since then. Reptiles are a mish-mash of different mechanisms.

If TSD came first, then the real question is not why did it evolve, but why is it still around. And the answer is likely because it works well enough. I couldn't find a lot of evidence that females make varying choices for next temperature, so in "real life" it might mostly be that plenty of males and females are always born. There is also recent evidence that genes affect when the temperature has an effect, so again, temperature alone is unlikely to drive sex ratios.

As far as possible advantages of TSD there is no definitive explanation that has held up to testing. It is possible that ideal female development has slightly different requirements than male development, so nest location choice by the parent may act to produce more females if the current environment supports them more than males. For instance, it has been shown that having more female offspring when temperatures are higher results in larger females.

Sorry to not have a simple answer for you. The whole idea is fascinating, along with other environmental triggers to development, like worker ant larvae becoming a winged queen if given the right nutrition, or butterflies that add "eyespot" pigmentation to their wings in wet years with increased predators.