The first mic ever hung upside down was the Telefunken/Neumann U47 and it was hung upside down for a reason. The M7 capsule in that mic had a diaphragm made from PVC, which starts drying out pretty much as soon as its poured. The diaphragm cracks like a spider web and becomes brittle and starts to lose the low end. Engineers were worried the heat from the tube was accelerating the process and began setting up the stand so that the tube was oriented above the diaphragm so the heat would rise up and away from it. Although in truth, more heat is generated by the mic's heater's wire wound drop resistor than the tube itself. Other benefits were discovered afterward, like it allows a music stand to be more accessible and forces singers to sing with their head lifted, which opens the throat up, captures more deepness from the chest and just gets a better performance.
It was really just a necessity that became a trend.