Interactions between the surface and the bulk in a strong topological insulator (TI) cause a finite lifetime of the topological surface states (TSSs) as shown by the recent experiments. On the other hand, interactions also induce unitary processes, which, in the presence of anisotropy and the spin-orbit coupling (SOC), can induce nontrivial effects on the spin texture. Recently, such effects were observed experimentally in the Bi2X3 family, raising the question that the hexagonal warping (HW) may be linked with new spin-related anomalies. The most remarkable is the sixfold periodic canting of the in-plane spin vector. Here we show that this anomaly is the result of a triple cooperation between the interactions, the SOC, and the HW. To demonstrate it, we formulate the spin-off-diagonal self-energy. A unitary phase with an even symmetry develops in the latter and modulates the spin-1/2 vortex when the Fermi surface is anisotropic. When the anisotropy is provided by the HW, a sixfold in-plane spin canting is observed. Our theory suggests that the spin-canting anomaly in Bi2X3 is a strong evidence of the interactions.