發 爛 渣 not only means that he's very angry, but it implied that this guy has a bad temper, and using his anger to threaten others so that he can escape some questions or responsibility.
And this is a Cantonese Chinese phrase only, not appear in Mandarin.
I wanna know why 發脾四 too... maybe it just sounds funny?
And one more thing, the tone of 發爛渣 is originally "faat3 laan6 za2", but sometimes people say it as "faat3 laan1 za4" (爛渣 becomes tone of "laser").
Maybe pretenting to be an English word??