The Ukrainians got fed up with Viktor Yanukovych and his corrupt government so they voted him out.
That annoyed Putin so he hit back with the invasion of Crimea and then he really pushed his luck with a full scale invasion of Ukraine.
If you didn't know, consider what that means about your news sources. Even if it doesn't change your positions on Ukraine, the US overthrew a country and it wasn't big enough news that they made sure you knew it. It's just a random interview buried on CNN or whatever.
Here's a transcript of the US Assistant Secretary of State and the US Ambassador to Ukraine planning the coup. https://www.bbc.com/news/world-europe-26079957
Here's Obama saying he brokered the deal for the transfer of power, resulting in the democratically elected leader fleeing. https://cnnpressroom.blogs.cnn.com/2015/02/01/pres-obama-on-...