It's fairly simple. In the absence of much pressure outside the rocket (which is the case when the rocket is at a suitably high altitude), the hot gasses escaping from the rocket simply keep expanding. For a deeper look as to why this occurs, you will want to look into the kinetic theory of gases.

They glow either because they are still quite hot (very near the rocket) or because they are high enough up that they are illuminated by the sun even when the sun is below the horizon from the ground.