The workers are complaining about working conditions.

Literal

Worker [topic-は] working-conditions [to-に] complaint [object-を] grumbling.

不平をこぼす ('to grumble, complain') is an idiomatic expression — こぼす literally means 'to spill,' so complaints are 'spilled.' 不平 (ふへい, 'discontent, complaint'). ~ている marks ongoing grumbling.