I suppose it’s possible that as a moody teenager he ran away from home (the Resistance) and joined up with the wrong people for a while. In fact, I wouldn’t be surprised if that’s the canon answer eventually.
As someone who has read every single piece of canon media, I can confirm that’s literally the answer haha. There was a book released in August last year called “Poe Dameron: Free Fall” which detailed his spice-running career’s start and end. He initially got caught up in it because he was frustrated by how overprotective his father was of him after his mother’s death and how bored he was stuck in his boring small town on Yavin IV, so while in the only remotely interesting thing near his house, the local bar, he overheard some of the Kijimi Spice Runners discussing how they needed a new pilot (after the last one was killed) and he, in classic Poe Dameron fashion, acted like a hot-shot-best-pilot-in-the-galaxy and joined the crew as their new pilot.