Pokémon Black&White: Instant Catch?

I was playing my Pokémon black game last night trying to get my Pokédex filled so I could trade with some friends later this week. Anyway, I get into a tag wild battle with a Sunkern and a Pinsir just outside of Lacunosa Town. Anyway, send out my Pokémon and knock out the Sunkern since I had it. Weakened the Pinsir and threw a few Ultra Balls. On the 4th one though, it shook in mid air then dropped. It shook once on the ground then *click* was caught.

I'm not sure what I did, but it was like an instant catch... I was wondering if anyone else has noticed this and in what situation/against what Pokémon and what Pokémon they were using. The ones I sent out were a Lugia and a Kyurem. I thought it was the Pressure ability kicking in, but an hour later I had the same effect happen using my Samurott in the Great Chasm. I forgot what Pokémon I was fighting though... Sorry. :x

It's called a critical capture, i'm not sure as to what triggers it, however.

Ah that's cool. I was starting to think it was like use a pokemon with a type advantage against a wild pokemon, and it like intimidates it. Thanks for the links!