While not physically forbidden, a negative piezoelectric response – defined as an enhancement of polarization under compression – is rare in nature. However, this effect has recently been found in several van der Waals bonded materials, among them the polar BiTe (=I, Cl, Br) compounds studied here. First-principles calculations reveal that their negative piezoelectric response is mainly induced by a charge-center shift of the Wannier functions (shown in the figure) under compression, especially on the halide, increasing the layer dipole moment and thus the polarization.