  We examined the shape staggering of relative charge radii in $^{180 - 186}$Hg
isotopes, which was first measured in 1977 and recently confirmed using
advanced spectroscopy techniques. To understand the nuclear structure
underlying this phenomenon, we employed the deformed relativistic
Hartree-Bogoliubov theory in continuum (DRHBc). Our analysis revealed that the
shape staggering can be attributed to nuclear shape transition in the Hg
isotopes. Specifically, we demonstrated that prolate shapes of
$^{181,183,185}$Hg lead to an increase in the charge radii compared to oblate
shapes of $^{180,182,184,186}$Hg isotopes. We explained the nuclear shape
staggering in terms of the evolution of occupation probability (OP) of $\nu 1
i_{13/2}$, $\nu 1 h_{9/2}$, $\pi 1 h_{9/2}$, and $\pi 3 s_{1/2}$ states.
Additionally, we clarified the kink structure of the charge radii in the Hg
isotopes near $N = 126$ magic shell does not come from the change of the OP of
$\pi 1 h_{9/2}$ state, but mainly by the increase of the OPs of $\nu 1
i_{11/2}$ and $\nu 2 g_{9/2}$ states.
