He didn't flub it. Read his quote, courtesy of ASAP sports:

QUESTION
"Talk about 16. I know you'll probably be thinking about that tonight."
CHAD CAMPBELL: Yeah, it was really a great shot. It was the wrong club. (Laughter).
It's just a mistake you can't really make. That's the only place on that hole you can't hit it. You can make par anywhere else, but that pin, you don't even know the water is there. I don't really know what happened. The thing just took off. Like the wind just laid down -- the wind just ate Kenny's up. Obviously didn't hit mine at all.
You get it back there, and I was hitting that shot, I was actually trying to -- I mean, there was probably about ten percent chance of me landing it in that first cut. If you don't land it in that first cut and you land it on the green, it could go in the water.
So I didn't -- it's hard to explain this. I was trying to land it in that little area, but my miss was to keep it in the bunker, so that's kind of why I didn't hit it very hard. You know, you get it on the upslope, you can keep it on that shelf.