Spoofer calls line 1 from line 2

Has anyone else ever seen this: I have 2 lines. Same area code but different exchanges. I received a call on line 1 and the CID said it was from my line 2. This is a small PBX, and after reviewing logs I know the call didn't originate from me. How the #!%% did they figure out to make a call like this? The chances of them just accidentally  calling my 2nd line while spoofing the first seems astronomical.

You can't assume that the spoofing caller was aware that you controlled both lines. I see a different scenario. Spoofer selects your Line #2 -- tag, you're it -- to be the regional decoy number of the hour or day, then sweeps a few dozen or thousand people across your state. Your Line #1 was merely caught in the dragnet.