A coup
The next election was won through the popularity of a tv series, where the election winner played president on tv. This was not a coincidence.
It became very clear that the policies of these two presidents that it would completely isolate Ukraine from Russia, despite half of Ukraine being Russian and the other half semi-Russian.
This upset parts of Ukraine that have deep economic ties with Russia so much, that they declared independence of Ukraine to join Russia, starting with Crimea.
This in turn was painted by US/UK media as aggressive annexation by Putin.
It also became clear that Zelensky wanted to join the EU at all costs.
This concerned Russia as the EU doesn't seem to be independent of NATO policy and having nukes placed close to Russia's border aimed at Moscow in areas where xx% of the population would consider themselves Russian wanting to join Russia is considered a nightmare scenario for Russia if separatist conflict would be taking place there.
Russia then made negotiations with Ukraine to address both concerns and Ukraine did not uphold their part of the agreement. This happened twice and in both cases this ended up with military victory for Russia and the separatist areas.
Russia also negotiated with NATO, since NATO was supplying Ukraine with arms to end this conflict and it became clear that NATO and the EU were pushing Ukraine to further the conflict.
At this point Russia decided that it was time to act before things would get further out of control, showing Ukraine's president that they were willing to wage war in order to prevent high otential nuclear conflict, by doing a small flash invasion, while preparing for world war III.
Ukraine's president then flipped, saying he was willing to take the situation serious, but then flopped when UK's former prime minister came to visit him telling Zelensky that NATO was prepared to fully support Ukraine with whatever military equipment he needed.