The unicameral National Assembly of Bulgaria has 240 seats of which the center-right Citizens for European Development of Bulgaria party holds 117. This has held steady since the last parliamentary election in 2009. Another thing that has held steady is the unemployment rate in Bulgaria since the start of the global economic downturn.
How did this lead to the fall of the Borisov government?
Bulgaria did not sidestep the effects of the global recession. Its unemployment rate stands at 11%. It would be much higher if not for the high rate of emigration from Bulgaria to other parts of the European Union. Those who remain are more and more upset at corruption, high energy prices, and rolling blackouts. This last sparked the last in a series of protests against the government.
Borisov's tenure as Prime Minister has not been easy. He has survived four no-confidence votes (article: Borisov Survives). His exit from the premiership was not due to a successful no-confidence vote, however. He voluntarily resigned in protest of the police beating protesters. At least, that is what he said.
Of course it may have something to do with his failed attempts to govern.
Borisov came to power with promises of eliminating corruption and raising living standards. The former continues. The latter remains low; Bulgaria is the European Union's poorest member. Raising power prices did not help Borisov, either. This action sparked the protests.
With the Prime Minister's resignation, expect early elections in Bulgaria, perhaps as early as April. Between now and then, Borisov's GERB party will be a caretaker government, merely ensuring that the National Assembly does its job.