I've experienced that on some tracks there's some kind of obstacle sticking out on the very right or the very left side of the track, right where the start/finish line is. If you go too far right or left while crossing the line you'll hit it and this usually ends in an unexpected and devastating disaster - like your ship suddenly flying nose first straight up into the sky at incredible speed, getting turned around 90° or even 180° or whatever nightmare can happen to an AG pilot who is totally focused on his race and therefore absolutely high on adrenalin.