Continuity mistake: When McGarrett and Danno question Frank's daughter, she has her hands clasped in front of her. When the shot cuts, her arms are suddenly extended down at her sides instead. (00:29:50)
Revealing mistake: When McGarrett and the modern-day posse find Frank's euthanized horse, the "dead" animal is quite visibly (and vigorously) breathing. (00:45:20)
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.