Continuity mistake: Danno drives away from 5-0 headquarters in a 1972 Ford LTD, but arrives at the crime scene in a '73 with a distinctively different front grill. (00:04:05)
Other mistake: The coroner says he strangled the women with his right hand. When he goes to strangle the final victim he uses his left.
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.