...And I Want Some Candy and a Gun That Shoots - S4-E6
Continuity mistake: Danno boards the helicopter wearing a brown suit, but arrives wearing a Hawaiian shirt. When he gets out of the aircraft a few shots later, though, he's back in the brown suit, with no time to have changed.
Answer: He was fired. He never really appreciated his character. He felt Kono was portrayed as a big dumb Hawaiian and that the stereotype was racist. He also felt underutilized. He was fired after a heated argument with the show's publicist regarding his character. It seems there's not much details given regarding the incident or the firing, so it's seems possible he upset the show's producers as well.
Bishop73
No he said something derogatory about one of the Jewish producers that's why he was fired.