You first need to find some butt morphs which will suits your needs.
Maybe something like "glute up" or "glute in"...
Maybe you will find some in the build-in morphs, but there are several more custom butt morphs around to download.
Next is to switch those morphs on/off by some sort of trigger.
-Most simple solution would be a collision trigger.
-A bit more complicated: a rythm trigger with sound source and music.
-You can also use a plugin to select and apply the morphs randomly.
-Or most complicated: you could use the Timeline plugin to create an animation plus switching those morphs.
Here is a very good tutorial on how to use triggers: