Why is it called CASVB?

A word that is formed by merging parts of two or more other words can be called a portmanteau. The much read author Lewis Carroll (real name Charles Lutwidge Dodgson) made extensive use of such portmanteaux. For example, he used 'slithy' to means 'lithe and slimy', with two meanings packed up in one word.

Remembering that 'CASSCF' is the standard abbreviation for complete active space self-consistent field, and that 'SCGVB' has been used as an abbrevation for spin-coupled generalized valence bond, the name 'CASVB' comes about as if you started by saying 'CASSCF' but, part way through, decided that what you had really wanted to say is 'SCGVB'.