Why does my cat do this....?

[Jen the Cat Captain]

My cat is 6 months old, so he's still little. At random times, he finds a toy and kind of..dances around it. Kind of like he's going to pounce, except he's up on his back legs, hopping around...
Also, when I'm standing around at the counter, he'll be sitting calmly next to me, then next thing I know, he does a little twittering meow and jumps up on my leg, hugging around it with all four paws.
Sometimes he'll be just sitting there, staring out the window, or looking at a toy, and he'll randomly do the twittering meow, stand up, and do this kind of twisting back flip thing. I call him the 'Matrix Cat' when he does it.
He also does this when he's play-fighting with his 'brother', my 9 month old cat. The 9 month old is bigger than him, so the little guy gets up on his back legs, and hops right at him, THEN pounces.
He does the little twitter-meow every time.

Is there a certain reason why he does this? He's very confusing...