An improbable battery fix

I have a 2002 Mazda Protege.

It's getting pretty old. Last September I got the alternator replaced at the local Mazda dealership, they also replaced the battery. Prior to that fix, I had a lot of issues starting the car, and picked up a set of self-jumper cables (regular red and black clamps on one end, a USB-rechargable battery on the other, so that I can jumpstart without the help of another car).

In the past couple weeks I have been often unable to start the car so I pulled out the jumpstarter again. To my surprise, I found all that I need to start the car is to only attach the black clamp to the negative terminal of the battery. I don't even need to connect the USB battery or the red clamp to the positive terminal, and the car will start fine as long as the black clamp is on. This solution has worked 3 times now.

This feels like a very weird problem where the laws of electricity are broken. Can someone explain to me why this is happening, and if there is a simple fix to avoid needing to open the hood for this reason again?