The pro European area was part of the Polish-Lithuanian commonwealth. Basically that's all the old Russians (now referred to as Ukrainians) who got conquered by the Poles.
The bits that Poland owned speak Ukrainian, the other bits speak Russian. Ukrainian and Russian were once the same language. They diverged mainly because the people lived in different political entities.
All the protests and pro-EU stuff is from the part of the Country Poland used to own. Next look at the 2010 presidential election results. Yanukovych, pro-Russian president who had to flee for his life during the protests won the bits that speak Russian...
Ukranian land outside of the Polish borders was conquered & settled by Russia from the Ottoman Turks and their Crimean Tatar vassals.
You get the gist