The Ukrainian policies against ethnic Russians pretty much start with formation of Ukraine and got worse over time, including language restrictions and economic policies. Then the NATO thing, we've discovered is some kind of sham, would be a real security issue if it were real, and we can't ignore the 15K people killed in Donbas, most of which were the reason Ukraine had 211 human rights violations in International Court. The chemical weaons labs just make the thing worse and possibly intelligence related to that could have forced their hands. I'm not entirely sure that Putin should have let himself be provoked, but I believe firmly there were attempts to do so.