"He is getting off."
Translation:Er steigt aus.
I suspect the "steigen" here is the old Norse meaning of "to step", which has the second meaning of "to rise". This usage as "to step" remains today as "stega" in archaic-sounding Scandinavian, with the only modern usage remaining as pacing the number of steps between two points ("Att stega avstånd", which may or may not be correct German as "Abstände zu steigen").
Only he's getting off of something that has no roof (inferring from Scandinavian branches of old German). You can get off from a horse, for example. Otherwise, you're getting out of it (aus).
You're getting off of a horse, but out of a [roofed] stagecoach, the way these branches of German see it.