Vucic literally spent a year illegally registering people from outside of Belgrade (including Serbs from Bosnia and Croatia) to Belgrade addresses and bussed them on the election day to vote, just so he could narrowly win local elections. That is the main outrage. These protests aren't even organized by opposition parties, people are just that pissed.

SNS gained prominence and became the largest opposition party due to their anti-corruption platform and the protests in 2011 at which they demanded early elections.

How the turntables.

Source

OOTL