Why on earth did he have to turn himself in and was not arrested on the scene? If they know his BAC at the time of the accident, then they must have had him in custody at the scene.
I suspect he was arrested at the scene on the drunk driving charge and bailed himself out. The prosecutor would have conducted an investigation over the following weeks and gotten a warrant on the negligent homocide charge.
The FHP gets one chance to file on an accident, they wanted the final results from the lab on Blood Alchohol level before filing. That's why he's getting the manslaughter charge. At least he turned himself in and didn't flee to Mexico. A very sad thing. The person killed was a local musician/surfer in his 40's.