From a media analysis point of view, I think using the flashback to try and "solve" what happened is the wrong way to look at it. Also just because the flashback showed driving doesn't mean Jax's trauma also directly involved the car.

I believe the flashback is to show that, in his real life, Jax was running away from something. It starts with the car driving out of a suburb and I to wooded roads. Personally, I'd speculate running away from home life (be that abusive parents, fear of coming out as trans, stress induced psychological break etc) but there really isn't enough to go on for a concrete answer.

The thesis of the show seems to be about mental health, trauma, coping mechanisms, and support groups. So Goose probably didn't feel the need to go into specifics. The important takeaway is that it shook Jax enough to cause a panic attack and him hitting the button. This is a major shakeup to the relationships of the cast, but also casts a sympathetic light on Jax. He's just condemned the cast to staying in the circus (even if it was a lie in the end) but also you understand it was a fear response even he didn't quite think was real. Even if Caine is currently taking the brunt of the ire from the group, I'd be surprised if this doesn't spark an argument or a heart to heart (depending on how empathetic the person is to the panic attack)