Ukraine didn't start the war, but definitely made quite a few wide steps toward the spillover of the conflict. To wit, Ukrainian speakers, think-tankers, and military folk repeatedly and openly boasted about their intention to solve Donbass frozen crisis following the Operation Storm’s example.

The main reason, in short words, is the violation of the principle of neutrality as written in the Declaration of State Sovereignty of Ukraine (1990). The declaration proclaimed that the republic has to become in a future a permanently neutral state that does not participate in military blocs. Instead it was slowly but inexorably turned into a brander ship, loaded with explosives and aimed at Russia to set it ablaze. So the invasion was a mistake from Russian side, though definitely not an unprovoked one. You may notice that a platitude about “unprovoked invasion,” which von der Leyen or Blinken were very fond of, has been dropped by the US administration at once as soon as talks with Russia began.