  We analyze extensive spectroscopic and photometric data of the hypervariable
quasar SDSS J131424+530527 (RMID 017) at z=0.456, an optical "changing look"
quasar from the Sloan Digital Sky Survey Reverberation Mapping project that
increased in optical luminosity by a factor of 10 between 2014 and 2017. The
observed broad emission lines all respond in luminosity and width to the
changing optical continuum, as expected for photoionization in a stratified,
virialized broad emission line region. The luminosity changes therefore result
from intrinsic changes in accretion power rather than variable obscuration. The
variability is continuous and apparently stochastic, disfavoring an origin as a
discrete event such as a tidal disruption flare or microlensing event. It is
coordinated on day timescales with blue leading red, consistent with
reprocessing powering the entire optical SED. We show that this process cannot
work in a standard thin disk geometry on energetic grounds, and would instead
require a large covering factor reprocessor. Disk instability models could
potentially also explain the data, provided that the instability sets in near
the inner radius of a geometrically thick accretion disk.
