  The anomalous low-energy $E2$-related behavior of a triaxially-deformed
nucleus bas been identified and analyzed based on the SU(3) algebraic theory
within the framework of the interacting boson model. The results show striking
features that include a
$B(E2;4_1^+\rightarrow2_1^+)/B(2_1^+\rightarrow0_1^+)<1.0$ transition rate
ratio and a $E(4_1^+)/E(2_1^+)>2.0$ excitation energy ratio that can be tracked
back to a finite-$N$ effect, which in a large-$N$ limit of the theory yields
normal results for a stable $\gamma$-deformation. This description is shown to
be able to explain observed $E2$ anomalous phenomenon in neutron-deficient
nuclei such as $^{172}$Pt and $^{168}$Os, and in so doing yields a deeper
understanding of the physical features of a soft triaxially-deformed nucleus.
