What was happening on this track?

4:14 into the Suspicious Minds track on FTD From Sunset to Vegas.

Elvis starts "singing" a waltz and then finishes off by laughing and saying (presumably to Charlie Hodge) "how much did you pay her to do that!"

What's happening on stage that causes Elvis to react this way?

4:14 into the Suspicious Minds track on FTD From Sunset to Vegas.

Elvis starts "singing" a waltz and then finishes off by laughing and saying (presumably to Charlie Hodge) "how much did you pay her to do that!"

What's happening on stage that causes Elvis to react this way?

What you are referring to is a bit of a very famous waltz by the Austrian composer Johann Strauss II, "The Blue Danube":

From Sunset To Vegas (FTD): Bonus Tracks
August 22, 1974 MS
Introduces John O 'Grady / Suspicious Minds
The Blue Danube (instrumental)

It's unclear what's happening, although Presley may be referring to Kathy Westmoreland, or one of the Sweet Inspirations, moving to the indelible melody.