Modulation via a major chord, then another major chord a half step down

Question

In Nick Cave's "Darker With The Day", there's a modulation at the end of the chorus which sounds very good (to me), but I don't understand why it works.

The verse is more or less in G major:

GM GMb7 CM Cm

The chorus is in Bb major:

BbM FM Gm FM EbM DM

...and then back to GM starting the next verse. The EbM to DM part is the part I'm wondering about. Why does that make so much sense to my ear? It sort of echoes the "liturgical" sound of the V -> v -> I thing from the verse; is that why?

Outside of this context, it doesn't usually sound all that great to follow a major chord with another major chord one half step down.

Answer 1

The D major chord is the dominant chord (V) of G major with the next chord being G major it makes sense this modulation would work. Typically in Bb major, you would build a D minor chord instead, but it's not unheard of to use different quality of chords especially when you want to change what chords you emphasis(like utilizing secondary dominants) or modulate. If a D minor chord was used before the D major chord then it could be considered a chromatic median modulation as the quality changing from minor to major fits that description perfectly.

Here is how I would look at it:

Bb major: I  V vi  IV (III or V/ii)
G  major:                  V              I

As you can see the progression starts out squarely in Bb major, but on the D major chord which can be looked as a a chord used to pivot back to G major.

Answer 2

If you're still wondering about the chord progression of two major chords where the second one is one half-step lower and the second one also functions as the dominant chord of a tonality, you may want to look into augmented sixth chords, which you can read about here. In your case, the Eb Maj chord would be similar to a German sixth chord in G minor. Though the chord is missing the distinctive raised scale degree four (i.e. C# in G minor) the Eb Maj chord still functions like a German augmented sixth in that it moves to the dominant chord. This is an interesting way to modulate from Bb major to G minor because it uses Bb major's IV chord (the Eb chord) in a context that only really makes sense in a G key.