The place was called the brook Eshcol, because of the cluster. It is very probable that it was already known as the valley of Eshcol, from the friend of Abraham, who bore that name and lived in that neighbourhood (Genesis 14:13, Genesis 14:24).
If so it is an admirable instance of the loose way in which etymologies are treated in the Old Testament: what the place really received was not a new name, but a new signification to the old name; but this appeared all one in the eyes of the sacred writer.