What caused an old, giant star to disappear for over half a year?

In the scheme of galactic magic tricks, one of the more difficult is making an entire star disappear. Especially with the added difficulty of making it reappear again a little over six months later.

Yet this is apparently what happened to a star halfway across the galaxy from us. In 2012, the star slowly faded over a few months, dropping in brightness by an astonishing 97% (that that, Betelgeuse!) before rising again at the same rate it faded. A drop like that is extremely weird, and very diffi...