It was a US backed coup, followed by Ukraine civil war, where Russia backed the ethnic Russian separatists in the East. Note that exactly this scenario was predicted by current CIA director William Burns (then US ambassador to Russia).

Here from an article from 2014 with more quotes from Burns.

"NATO enlargement, particularly to Ukraine, remains ‘an emotional and neuralgic’ issue for Russia, but strategic policy considerations also underlie strong opposition to NATO membership for Ukraine and Georgia. In Ukraine, these include fears that the issue could potentially split the country in two, leading to violence or even, some claim, civil war, which would force Russia to decide whether to intervene," the cable read.

Link to 2014 Business Insider article.

https://www.businessinsider.com/wikileaks-russia-cable-2014-5