6th Gen Critical Capture?

Let me begin with saying that I never owned a Gen V game for various reasons, including lost ownership of my DS. However, I recently bought a 3DS and Pokemon X. I whizzed through the main game and eventually found myself at the end of Terminus Cave. After putting Zygarde to sleep and weakening it quite a bit, I threw a few Ultra Balls and, much to my surprise, the ball shook but once and darkened. I researched this phenomenon and it is apparently called a "Critical Capture". Does anyone know the chance of this happening? I was kind of startled that I may have accidentally selected my Master Ball after it happened, haha. Feel free to share any other Critical Capture stories.

I've come across critical captures in both 5th and 6th Gen, but I heard the chances of getting one increase as you catch more Pokemon. It's not exactly known how often a critical capture occurs but they're usually distinguished by a "clang" sound when the Poke Ball is thrown, and the Poke Ball hovers in mid-air for a bit before falling, plus it takes only one shake instead of the usual three.

There is a small chance critical captures can fail, but most of the time it does land a successful capture. I was happy to see it retained, it was a pretty nice little feature in my opinion.

It occurs both in Gen V and VI. From what I read, it depends on the amount of Pokemon species you have captured. I think Serebii has a page on it with the formula used. You will know when it occurs. When the trainer throws the Pokeball, there is a distinct whooshing/clang sound. This actually happens a lot to me when I encounter a shiny using the PokeRadar. I throw a Quick Ball on the first turn and get a critical capture.

Like Hikamaru said, there is a small chance of this failing.
I also like this feature, it gives me the feeling that the Pokemon wants to come along with me on my journey. :D