This answer is setting out to pinpoint the main causes of the Russian invasion of Ukraine.
The long-term cause of the Russian invasion of Ukraine seems to have been the failed promise of the Americans to not extend NATO past Germany which happened in 1999 when the Czech Republic, Hungary, and Poland became members.
A long-term cause that intensified in 2004 when seven other former Soviet Union countries became members, leading to Ukraine’s increased interest in joining NATO. For Russia to earlier have been given assurance that NATO would not expand towards the east, to now have NATO so close to its border was completely unacceptable, and Russia tried therefore to woo Ukraine to not jump onto the bandwagon but to stay neutral.
Something that was not welcomed by Ukraine, who in February 2019 enshrined in their constitution the goal of becoming full fledged members of NATO.
The short term cause of the Russian invasion of Ukraine seems to stem from about 2015 when they first could have perceived a potential biological weapons threat from the bio-labs of Ukraine. What seemed to have worried them most was that it was their old nemesis the United States that was funding these bio-labs. A real threat or not, having lived through the massive Chernobyl catastrophe first hand, Russia would have earned the right to be a bit paranoid in this area.
(A special caution to the Russians needs to be added here. As was hinted at in the airport customs scene, at the end of the film "12 Monkeys", deliberate shelling of a city could backfire, with similar or worse consequences than what the Chernobyl accident had. Because, frozen dangerous pathogens could thaw and escape from a damaged bio-lab, if freezer power is cut off. Indeed, the Geneva Protocol on just warfare (part 4) requires that only the military be targeted, not civilians.)
The trigger of the Russian invasion of Ukraine seems to have been the failed Minsk II Accord (12 Feb 2015 – 21 Feb 2022), which Russia had hoped would bring peace to the unstable Donbass region, where Ukrainian fighters were attacking the Russian-speaking separatist states: Donetsk and Luhansk.
The failed relationship between Ukraine and Donbass may be summarized in the words of a novelist: “Donna filed for divorce from her abusive husband Kerry after eight years of unsuccessful counselling. She then also took out a restraining order against him with the local authorities".
Consequently, in order of importance, the causes behind Russia’s invasion of Ukraine seems to have been: 1. The continuous bombardment by West and Central Ukraine of the Russian speaking separatists in East Ukraine. 2. Ukraine’s legislated aim to join NATO. And, 3. The reports about US funded, potential accidental biological spills, bio labs in Ukraine, alarmingly close to Russia.