One possibility, for which there is abundant evidence, is that the antibodies are antibodies directed against cellular proteins. In other words, autoantibodies. This is a list of a few of the autoantibodies that have been found in the sera of AIDS patients. There’s obviously a considerable number but a MEDLINE search will reveal many more. And the list keeps growing.

But there are also other reasons which have nothing to do with retroviruses to account for a positive Western blot.