I've posted this before. Not so much to do with the current style and benefits of hanging a tube mic upside down, but the original reason happened because if the Telefunken U47. The M7 capsule in that mic was made from poured PVC, which basically started drying out from the moment it was poured. After long term drying out, the diaphragms become brittle and crack like a spider web, causing the mic to lose all low end or even cut out entirely. In the U47 two sources of heat (the VF14 tube and a huge wire-wound resistor) caused so much heat that in the right-side-up position engineers were worried it would accelerate the process, so they turned the mic over to keep the capsule from drying out faster than normal. It was a misunderstood usage and caught on as a trend for hanging mics upside down.