Russian deployment follows decision to recognise territories in south-east will be viewed in Ukraine and by other western allies as an occupation
www.theguardian.com
Putin has recognised the Donetsk and Luhansk "People's Republics", the breakaway separatist regions in Donbas, in contravention of the Minsk Accords.
The Minsk Accords aimed to reintegrate the regions into Ukrainian control, and were signed by Ukraine, Russia, and the OSCE, as well as the DPR and LPR themselves.
This lines up with the fear that the breakaway quasi-states could provide the basis for invasion and annexation. The OSCE has recorded hundreds of ceasefire violations in Donbas by both sides. It's also very reminiscent of what happened in Crimea in 2014: Russia claimed Ukrainian aggression; then refused to comply with international observers who might have verified it (just as in September 2021, they withdrew from OSCE); then invaded and annexed Crimea with that aggression as the pretence.