The perceived (or quite possibly real) difference between “western” values of how West understands democracy and human justice and “eastern” values of ethnic brotherhood and loyalty to “Mother Russia”.
Ukraine somehow became a battlefield between “East” and “West”
The political process in Ukraine got hijacked by extreme nationalists (more about that here) from Western Ukraine who want to assert their political agenda of subjugating the Russian leaning population and force them to become “true blue and yellow” Ukrainians and not “Moskali” (which means supporting Moscow). So, Russian-leaning Ukrainians resist as people always resist if somebody pushes anything down their throat. Crimea (read more about Crimea’s history) historically was a Russian territory, as was most of Ukraine, so it is easy to see why there would be such loyalty to Russia.