Question: Ceiling Fan - Breaker Trip

I may not be in the right sub reddit, if so, I apologize and would ask to be gently nudged in the right direction.

I just hung two ceiling fans in my apartment. The first install went relatively well. The second seemed to go even better; except that, when I turned the breaker back on, I got an instant trip. Nothing unusual observed or smelled.

I disassembled the fan install, piece by piece and continued to get the trips until I was down to wires and an electrical box.

I checked my wiring, checked the wires with my meter, and it all checked out.

I then reinstalled the fan, step by step, checking the breaker at each step, and it is now fully-installed, with no problem. Only difference, we didn't tighten it quite as tightly to the electrical box this time.

We never found the fault (if there was one). How concerned should I be here?

Thank you in advance.

PS-it is possible that it was operator error flipping the breaker (I am aware it's just a switch). Every time it failed, the breaker was flipped by my father, but he did flip it successfully prior to the string of trips. -not sure if relevant, but providing the info.