  NGC 5548 has been intensively monitored by the AGN Space Telescope and
Optical Reverberation Mapping collaboration. Approximately after half of the
light curves, the correlation between the broad emission lines and the
lag-corrected ultraviolet continua becomes weak. This anomalous behavior is
accompanied by an increase of soft X-ray emission. We propose a simple model to
understand this anomalous behavior, i.e., the corona might fall down, thereby
increasing the covering fraction of the inner disk. Therefore, X-ray and
extreme ultraviolet emission suffer from spectral variations. The ultraviolet
continua variations are driven by both X-ray and extreme ultraviolet
variations. Consequently, the spectral variability induced by the falling
corona would dilute the correlation between the broad emission lines and the
ultraviolet continua. Our model can explain many additional observational
facts, including the dependence of the anomalous behavior on velocity and
ionization energy. We also show that the time lag and correlation between the
X-ray and the ultraviolet variations change as NGC 5548 displays the anomalous
behavior. The time lag is dramatically longer than the expectation from disk
reprocessing if the anomalous behavior is properly excluded. During the
anomalous state, the time lag approaches the light-travel timescale of disk
reprocessing albeit with a much weaker correlation. We speculate that the time
lag in the normal state is caused by reprocessing of the broad line region gas.
As NGC 5548 enters the abnormal state, the contribution of the broad line
region gas is smaller; the time lag reflects disk reprocessing. We also discuss
alternative scenarios.
