Why is the event called the supper of the great God?
It is called the supper of the great God because the scavenger birds and birds of prey are summoned to feast on the flesh of the assembled armies after the great conflagration, serving as a cleansing effect that prepares the earth for the soon appearance of the kingdom.
Les explains that the pronoun in this passage points to the birds—vultures and scavenger birds flying in the atmosphere—not birds in the throne room. They are told to gather to the supper of the great God and eat the flesh of kings, captains, mighty men, horses, riders, and all men, free and bond, small and great. This great conflagration falls upon the assembled armies, especially in the Middle East, and these birds of prey have a cleansing effect, getting the earth ready for the soon appearance of the kingdom. Les says all the Old Testament speaks of this event when scavenger birds feast on the gathered armies, and that is why it is called the supper of the great God. He also mentions a reported detail about a Middle Eastern scavenger bird multiplying at almost double normal, though he treats such reports cautiously.