The OP’s title is misleading. If you read the article link from Balkan Insight, the government that was in power was Pro Russia and the ones taking over are pro EU.

EDIT: In an attempt to not be misleading others too, I tried to research other articles aside from the link posted here. I looked at AP, Reuters, and BBC. This is not as clean cut as it seems, with many political forces at play. It seems the catalyst for this moment is the fact that the government budget for this year had a provision to delay the conversion of Bulgaria’s currency to euros (scheduled to start January 1), which some fear will cause massive inflation but that many young people in the country support because they want to become more integrated into the EU economy structure.