Complex sentence: In a post-credits scene, the Minions and almost everyone from the film do a dance party, singing The Beatles' "Revolution" in Minionese, however everyone except Young Gru runs off when the T-Rex from the opening of the film shows up.
Split into simpler sentences: In a post-credits scene, the Minions and almost everyone from the film do a dance party. They all leave and Young Gru gets left behind when the T-rex from the opening of the film shows up.