what explains this division?
Western Ukraine, Belarus and the Dialect Region of Russia correspond to the medieval Rus. Back in the 19th century it was «Russia's England»: Here the dialects were thick. But after the Empire went down two of those dialects got recognized as independent languages.
The nationalist movement went through a long struggle for recognition in the Austrohungarian Empire. They say that Ukraine is a nation exactly because of it's Ukrainian language which has to be spoken throughout the country much like the French language is universally spoken in France.
People in the East also consider themselves Ukrainian. They believe that whatever they speak is the Ukrainian language and they don't want to be forced to speak another Ukrainian language from the West. They do have their own dialects: in the East it is Surzhyk. In the South it is Odessan Russian. And they've been spoken there for centuries. Naturally they regard the nationalist movement as something oppressive and foreign.
In WW2 the Western Regions were first annexed by Stalin and then by Hitler. This is why they happen to regard Hitler as a liberator and celebrate collaborationists as heroes. Yuschenko's decision to build monuments to Stepan Bandera, an antisemite responsible for the genocide against Ukrainian Poles, was wildly popular in Lviv but looked bizarre and disgusting to the rest of the world. That's what got him voted out of office.
What is present-day Ukraine did not always have the geographic borders that it does today.
This wikipedia map here demonstrates it all pretty well.
Western Ukraine used to be part of Interwar Poland. The Ukrainian SSR got large swaths of southeastern Poland and a piece of Romania after WW2. These were areas that were historically under the control or influence of the Austro-Hungarian Empire and the Prussians (Germany). This map here also shows the administrative divisions of Ukraine under Imperial Russia, which at least in my opinion seems to fairly represent some of the present divides.
The East, especially the Donbas and Dnepropetrovsk region, was a major center of industry and coal mining in the Soviet Union. Although most of the population is ethnically Ukranian, they assimilated into the Soviet culture and became predominantly Russian speaking on the basis of the region's predominantly urban working class character. Many immigrants from all over the USSR came to the Donbas because coal mining was an extremely profitable, albeit difficult and dangerous profession, further diluting any Ukrainian nationalist sentiment.
The West of Ukraine, on the other hand, has always been rural, religious, and opposed to Soviet rule. It was never part of Russia's sphere of influence until 1939, when it was annexed into the Ukrainian Soviet Socialist republic after the Soviet invasion of Poland. Thus, the people there have a strong sense of national identity and independence. Instead of fighting for the Red Army, they formed their own armed group, called the UPA, which fought against Polish civilians, the USSR, and Nazi Germany: http://en.wikipedia.org/wiki/Ukrainian_Insurgent_Army
Many people in the West regard Stepan Bandera and the UPA as heroes, in spite of their horrific slaughter of 100,000+ Poles, Jews, Russians, and Soviet sympathizers. For Eastern Ukraine, which was always pro-Soviet, the veneration of the UPA in the West is extremely unsettling. A lot of the Berkut (riot police) are from the East, and you can see this sentiment reflected in their propaganda- they drew placards saying things like "Под Бандерой жить не будем, подвиг дедов не забудем" (Pod Banderoy zhit' ne budem- podvig dedov ne zabudem- We won't live under Bandera, we won't forget the struggle of our grandfathers).
There are a lot of regional tensions at play here. My relatives from Eastern Ukraine, for example, don't buy any produce produced in the West. Western politicians have publicly called the East of Ukraine to be cordoned off with barbed wire. The East views the West as uncivilized, violent peasants, and the West views the East as a shithole full of crime and urban decay. There is no real "Ukrainian nation"- there are two totally different cultural groups living under one state, which is never a good thing, and almost always leads to civil unrest.
That's why I think a split along linguistic lines is inevitable. The only alternative is prolonged and bloody civil conflict, which is in nobody's interest.
