This is the default for even-electron systems and for odd-electron systems that use keywords such as MECI that imply an RHF calculation. For odd electron (radical) systems, the UHF method is used by default, because geometry optimization using UHF runs faster than when RHF is used.
If RHF methods are to be used on odd electron systems, add keyword RHF.
RHF methods are needed if configuration interaction is used, see MECI.