The short answer is I don’t know. Perhaps the star’s photosphere has reduced in radius by ~30% (and hence halved in surface area) at approximately constant effective temperature ? The original Atel
http://www.astronomerstelegram.org/?read=13341
notes a small drop in Teff, indicated by an increase in the TiO band depths, which I am also seeing but the temperature drop quoted there does not seem to be sufficient on its own to explain the drop in luminosity. Since luminosity goes as T^4, the temperature would need to drop by ~%16 to halve the luminosity.