Short circuit between Vdd and Vss after high current through microcontroller clamping diode
By connecting an IO pin before connecting ground you caused an unknown (but clearly destructive) voltage to appear between the IO pin and the microcontroller's own ground voltage. If you intend to hot plug the microcontroller you need to protect the connections, preferably by connecting ground and power before the IO signals and also adding components to limit the voltage on the IO lines.

You have catastrophically damaged the microcontroller, so a short from Vdd to Vss would not be surprising. It could happen in several ways. Without knowing the exact structure of the microcontroller's input protection circuitry you would only be guessing at what happened. If you really want to know you will have to pay a failure analysis lab to investigate.