The President of the Ukraine was pushing for closer relations with Russia. Some people (mainly in western Ukraine and various oligarchs) were unhappy so they held a revolution. The US and most of western Europe sided with the revolutionaries (no surprise) since we wanted a pro-US government in Ukraine rather than a pro-Russian one. Unfortunately once all was said and done, the people of eastern Ukraine (who live in the Donbass region and who have always wanted closer ties to Russia) were understandably upset at the overthrow of the president they had elected. So they seceded from the rest of Ukraine to join Russia, and Russia decided to assist them.
Of course, there's more at play than simple politics - Donbass has some pretty massive oil deposits which (thanks to advances in fracking) can now be accessed far easier and cheaper than a few decades ago. Western oil companies had been "cut out of the loop" by Ukraine's government (who was offering better contracts to Russian oil companies). So those western oil companies had been lobbying US and other European governments to reject Ukraine's president and accept the new revolutionary government. Unfortunately for them, by the time all was said an done, the Donbass region and it's oil deposits are now under de facto Russian control.
There are lots of ways that this came together. As an example, then-Vice President Joe Biden's son, Hunter was working closely with Burisma Holdings (Ukraine's big oil company) which was unhappy with the Russian ties (since they felt that they could get better deals by working with western companies than the Russian government). Burisma Holdings initially came out on the side of the revolutionaries, though once it became clear that Donbass was going to be controlled by the Russians they started changing their tune. Still, Hunter Biden's dad, Joe, was one of the biggest pro-revolution voices in Washington D.C., gave lots of speeches about and applied a lot of pressure about how the US had to side with the Ukrainian revolutionary government, and it seems rather unlikely that his son didn't have some kind of influence on him.
There were, of course, tons of other factors in play, but in general I would say that Omega is correct in that the US (and other European nations and various oligarchs) pushed for the overthrow of Ukraine's elected government, because the government was seeking closer ties with Russia and the situation certainly has spiraled out of control since then.