  Divacancy spins in silicon carbide implement qubits with outstanding
characteristics and capabilities in an industrial semiconductor host. On the
other hand, there are still numerous open questions about the physics of
divacancy point defects, for instance, spin relaxation has not been thoroughly
studied yet. Here, we carry out a theoretical study on environmental spin
induced spin relaxation processes of divacancy qubits in 4H-SiC. We reveal all
the relevant magnetic field values where the longitudinal spin relaxation time
T$_1$ drops resonantly due to the coupling to either nuclear spins or electron
spins. We quantitatively analyze the dependence of the T$_1$ time on the
concentration of point defect spins and the applied magnetic field in the most
relevant cases and provide an analytical expression. We demonstrate that
dipolar spin relaxation plays a significant role both in as-grown and ion
implanted samples and it often limits the coherence time in 4H-SiC.
