Fascinating phenomena observed under applied pressure and magnetic field are attracting much research attention. Recent experiments have shown that application of the pressure or magnetic field to the USb₂ compound induce the transformations of the ground-state antiferromagnetic (AFM) structure (+-+) to, respectively, ferromagnetic (FM) or ferrimagnetic structures. Remarkably, the magnetic critical temperature of the FM state, induced by pressure, is more than two times smaller than the Néel temperature of the AFM ground state. We performed density-functional theory (DFT) and DFT+U studies to reveal the origin of the unusual magnetic ground state of the system and the driving mechanisms of the phase transitions.We investigate both the magnetic anisotropy properties and the parameters of the interatomic exchange interactions. To study pressure-induced effects we carry out calculations for reduced volume and demonstrate that the existence of the AFM-FM phase transformation depends on the peculiar features of the magnetic anisotropy. We discuss why the magnetic field that couples directly to the magnetic moments of atoms leads to the phase transition to the ferrimagnetic state whereas the pressure that does not couple directly to magnetic moments results in the FM structure. Our work demonstrates how the competition of different physical factors leads to variety of unusual properties of the antiferromagnetic USb₂.