  Recently, a new interesting instability of a charged scalar field in the
Reissner-Nordstr\"om-de Sitter background has been found (arXiv:1405.4931v2)
through the time-domain integration of the perturbation equation. We
investigate further properties of this instability, confirm its existence by
concordant frequency-domain and time-domain calculations and show that it
occurs at however small value of the coupling eQ, where e and Q are charges of
a scalar field and black hole respectively. We also investigate the parametric
region of instability and show that the critical values of eQ at which the
stabilization happens strongly depends on the value of cosmological constant
$\Lambda$ and softly on Q. We show that all the unstable modes are
superradiant, but not all the superradiant modes are unstable. We analytically
prove that superradiance is necessary (but not sufficient) condition for the
instability in this case and, thereby, demonstrate the superradiant origin of
the instability.
