Through the earthquake God “is reminded” (as the Greek actually has it) of what Babylon deserves, namely, the cup of the wine of the fury of his wrath.
So God gives her that cup with the intent that she must drink it. The detail of what happens when she drinks it will be revealed in Revelation 17. For now it is enough to recall that the “wine of God’s wrath” and “the cup of his anger” had received a mention earlier in Revelation 14:10. See commentary there.
19 The great city was split into three parts, and the cities of the nations fell, and God remembered Babylon the great, to make her drain the cup of the wine of the fury of his wrath.