Brazil itself grew a lot in that decade, so much of the growth can be attributed to simple population expansion. http://www.ibge.gov.br/english/presidencia/noticias/noticia_visualiza.php?id_noticia=1766&id_pagina=1
The rest of it is surely due to the post-Catholic effect. JWs and other evangelical faiths always mop up after catholicism, who are nearly always the first ones to pave the way for Christianity.