Does anyone actually know biologically WHY a dogs....?

Throughout pregnancy, there are two hormones that work in opposite ways on the uterus and the growing fetus.

Progesterone relaxes the uterus and keeps the fetus in your body, incidentally, it also raises the body temperature.

Estrogen stimulates the uterus and initiates rhythmical contractions, which happens during childbirth. It also lowers the body temperature.

Throughout pregnency, progesterones are dominant over estrogens, so the child stays in the uterus. However, as parturition nears, more estrogens are secreted than progestrones, until a few hours before the onset of contractions, there is a huge surge in estrogen and a drop in progesterone. This surge is enough to lower the body temperature a degree or so.

PS: In pets, body temperature is not always a good indicator of parturition because pets are often easily excitable (especially when they have a thermometer stuck up their butt!), causing wide fluctuations in body temperature even for a normal animal.

Sure. This is common in mammals. Usually a day or two before birth the animal stops eating. The temperature drop does several things. It signals to the dog to stop eating and it reduces biological demand and need for food, by moderating the dogs metabolism. Nature in effect is telling the animal to spend all its time and energy on giving birth.