I touched the negative cable to the negative battery terminal, it sparked and the headlights tried to turn on?

My truck was having trouble starting. It would drive fine, then I’d park it, go into a store, come out and try to start it. It would try really hard to start but just couldn’t crank over. It didn’t feel like a normal dead battery issue. I jumped it with a portable jump starter and it fired right up. After I got it home, I went outside to move my truck to the other side of the garage, and it wouldn’t start up again. This time, I jumped it with my wife’s vehicle. That jump lasted for a few days and then the same issues started happening again. I assumed it was a bad battery, so I bought a new battery. I pulled out the old battery and began installing the new one. I connected the positive cable first and went to connect the negative. I barely touched the negative cable to the negative battery terminal, it sparked, and the headlights tried to turn on. I checked to make sure all electronics in the truck were set to the off position. I tried connecting the negative cable again and got the same result. Is it safe to complete connecting the battery? Is this a grounding issue? A short somewhere in the wiring harness? An alternator issue? The truck is sitting in my garage with the battery completely disconnected because I’m not sure what to do from here.