DPR and LPR have been autonomous since 2014, the heads of the republics made a formal request for assistance from Russia in February after Russia recognized their independence rt.com/…/550394-donbass-republics-military-help-p…
This was intentionally modelled on the precedent NATO set on Yugoslavia where NATO recognized the independence of the breakaway regions and then had them invite NATO for assistance. Russia is just following the legal precedent set by the rules based order we keep hearing about.