We investigate an anomalous collective motion in atomic nuclei observed in recent experiments, characterized by three distinct signatures: (1) an excitation energy ratio [images] sizable electric-quadrupole transition strengths B(E2) in low-lying states, and (3) a suppressed ratio [images] This pattern challenges the prevailing understanding of nuclear theory because it contradicts all established collective models as well as microscopic models in nuclear physics. Using the full configuration-interaction shell model combined with the two-body random ensemble, we show that such an anomalous rotational mode emerges naturally in a small fraction of random many-body systems, providing critical insights into the connection between triaxiality and the emergent collectivity. The anomaly can arise from strong mixing between the 4+ states in the ground-state band and the triaxiality-induced γ band in the SU(3) dynamical symmetry. These findings underscore the necessity of incorporating triaxial degrees of freedom in descriptions of nuclear collectivity and the critical role of the residual two-body interactions in inducing the rare collective mode with mixed nature.