Why do the verb tenses change in this excerpt from The Ballad of the Sad Cafe by Carson McCullers?"The whiskey they drink that evening (two bottles...

I checked the original and your quote is not quite
precise, because it begins with "The whiskey they drank that evening...." However, I do
not think that is the verb shift you are asking about.

The
shift from the past tense to the present tense gives this passage an immediacy it would
not otherwise have.  It puts the reader in the moment, drinking that whiskey and
understanding its worth. The shift from the present tense to a future tense is a way of
holding out for the reader the idea of what could happen while he or she is sitting
there savoring the drink.

This is a beautifully written
passage, with a wonderful analogy to the magic one experiences the first time one tries
the lemon juice writing.  All is revealed by Miss Amelia's whiskey.  My favorite line,
though, is "a sweetness keen as pain."