Why did this Cirrus deploy the parachute while ditching over Pacific Ocean?
Once your engine fails the airplane is the property of the insurance company, your only goal at that point should be to survive. Using a whole-airframe chute increases your chances of survival whether over land or sea. 

The things that kill people in forced landings of any kind are high-g deceleration and post crash fire. A parachute decreases the chances of both as you have very low lateral speed and much less chance of fuel tank leaks. A parachute also will increase the chance the airplane will remain upright, so escaping the aircraft will be much easier - very important in a water landing!

The only reason for a "normal" ditching procedure is that you have no other choice, using a parachute if you have one will almost always be the right option. The only time I would consider otherwise is if I had enough altitude to glide to a safer emergency landing spot. If pulling the chute would mean landing in water or dangerous terrain but gliding would mean reaching land where a forced landing was reasonable I'd probably opt for gliding.