Negative battery terminal sparking on contact

Hello fabo! I just recently installed a Dakota mini starter on my 65 barracuda 273. I've had the negative battery terminal disconnected for a month while the car sat. After I installed the new starter I go to connect the battery terminal and to my surprise a very large spark results from the contact. Not only that, but I hear the starter try to jump from the brief connection. There was no key in the ignition.

Does anyone know what may have caused this? The positive from the battery shorting out somewhere? Maybe the starter relay is stuck? Any input is appreciated, thanks everyone.