Russia's claim of interest is to protect their Naval installation.
The Ukraine entered into a 25 year agreement to allow a Russian Naval Base on their soil.
2010 Kharkiv Pact - Wikipedia, the free encyclopedia
That move alarmed Ukrainians early in Yanukovych first term.
I think the last straw was that latest deal between Yanukovych and Putin to distanced the Ukraine from the EU for some bailout money, was seen as the first step of the Ukraine being sucked into the Eurasian Economic Union led by Russia.
Eurasian Union - Wikipedia, the free encyclopedia
Plus, in true Post Soviet style of political reign, Yanukovych made himself rich on the spoils of his office... while the country was collapsing economically.
That former president did a number on the Ukraine.