Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation and magnetic susceptibilitymaximum near 90 K, and, importantly for high-temperature magnetic applications, an increasingcoercivity (unique to MnBi) above 180  K. We calculate the total energy and magneto-anisotropyenergy (MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above anomalies. We predict that coercivity and MAE increase due to increasing a, suggesting means to improve MnBi permanent magnets.