Why does the Baron throw Candide out of his home? How does this plot event relate to the statement that this is “the best of all possible worlds”?

The Baron discovers Candide and Miss Cunégonde, the attractive daughter of the Baron, kissing behind a screen and banishes Candide from the castle. The castle’s tutor, Pangloss, teaches “metaphysico-theologo-cosmolo-nigology” . Pangloss believes that learning all three arts is the “best of all possible worlds.”