Russia invaded Crimea and stole it away from Ukraine in 2014. There were no talks about Ukraine joining NATO prior to 2014. The Ukraine conflict was provoked 100% by Russia, and not at all by the West. It started with the taking of Crimea. Then, Russia saw how easy that went and decided to take as much of Ukraine as it could get. Its as simple as that.
The bottom line - Ukraine had a very weak military. Having a weak military leads to stronger nations taking what they want.