Well, if we're going to go with what started this bout of unrest it would be the democratically elected President of Ukraine, Viktor Yanukovych, turning down an Association Agreement with the EU. That led to protests in certain major cities (notably the capital), riots (generally pitting the far-right fascist/neo-nazi elements of the opposition against the police) and the country essentially being split in two between the East that supported Yanukovych and the West that opposed him. An agreement was reached between Yanukovych and the opposition for a ceasefire and a way to proceed but the more hardline elements in the opposition continued their assault and forced him to flee. It's suspected there was pretty heavy EU and US involvement in funding and propping up the protests but I doubt we'll ever get to the bottom of that and leave it to the conspiracy theorists.
But what about the previous bouts of unrest?
One could look at the Gas dispute of 2006... which undoubtedly had wider issues at play but on at least one level was pretty simple; the Ukranian government wasn't paying for the gas it was using (essentially stealing it). One could point to the issue of gas debts from pretty much the breakup of the Soviet Union to the 2009 crisis.
One could point to the Orange Revolution of 2004 and see many of the same issues; a more European-minded West funded by Western Governments, pressure groups and unofficially exiled Russian Oligarchs against a more Russian-minded East funded by the Russian government. But it was kicked off by the murder of Georgiy Gongadze, a journalist who exposed corruption on all sides (and would have had a field day with many of those who made up the Orange Revolution).
Should we look back to the 2+4 negotiations and the Russian perception that NATO agreed to not expand any further east then Germany, only to see the start of expansion eastwards (including into Ukraine)?
Or should we point back to 1939-1945 where Ukraine was given fairly significant parts of Romania, Czechoslovakia and Poland, bringing its borders further West and bringing in large groups with less ties to Russia? Or 1954 where Crimea was transferred from Russia to Ukraine, bringing with it large groups who thought themselves Russian and had much closer ties to Russia?
This tension between East and West in Ukraine has been in existence pretty much throughout its history. One can look back to the Middle-Ages and see the way it was largely cut up between the Western-leaning Polish–Lithuanian Commonwealth, the Eastern-Leaning Crimean Khanate and the independently minded Zaporozhian Cossacks. The rise of Russia and the EU/USA as competing power-blocks only exacerbated the issue... and once Ukraine became the main route for natural gas from Russia into wider Europe it became even more significant. The country is, unfortunately, subject to both international economic and geo-political and domestic cultural and national identity issues. As is the way with such things, tragically they tend to occasionally boil over... which may be great for analysts, planners and strategists in offices far from the consequences, be they in Moscow, Brussels or Washington, but is certainly nowhere near as great for those who have to live with them day by day. Blaming any one side for this conflict... or in truth pretty much any of the conflicts over the last 20 years... is naive, as is absolving one side of blame.