The way I see it, Meiko and Kaito were playing 'The Matrix' when the force from one of Meiko's spin kicks flipped up Miku's dress (to which Kaito got a very good view of). During this time Len decided it would be a good idea to poke Rin on her butt. Just as Len was about to initiate this maneuver, with his eyes closed, he failed to notice Miku's leg poking out when Miku was distracted by her upskirt. In the meantime, Rin attempted to jump on Luka's bones cuz she's just cool like that, but failed to notice Len's little prank behind her. Since Len had his eyes closed, he not only did not see this but he also completely missed his target and jammed his fingers into Rin's 'special place'. Rin, in complete surprise, pulled down Luka's shorts. In short, Kaito and Meiko are too cool to obey the laws of gravity, Rin got an assfull, Len's gonna get a facefull (of fists and feminine fury), and we (as well as Kaito and those girls in the background) got an eyefull.
Here’s how I see it: For some unknown reason Rin decided it would be funny to "pants" Kaito. However after years of abuse Kaito naturally developed a type a "spider sense" and the reflexes to go with it. So when Rin lunged at Kaito he performs a well timed back flip making Rin accidentally pants Luka who happened to be standing in front of him. During this time Len who was standing way in the back had already decided deliver rectum wrecking "kanchou" to his sister. Being in the back of the group dashes forward to close the distance, but the tailwind of his run blows up Miku's dress. During mid-flip Kaito sees a stripped mountain and takes that time to admire the view. Meiko, who happens to be the only person Kaito's senses don’t work on noticed Kaito in the air and just decided to kick the shit out him "again" for no real reason.
You are all very close, but still far from it. You see, this all started with a prank by Len. He was attempting to prank Kaito, by performing kencho. Kaito knew this was coming, so he performs a back flip, dodging Len. With the combined efforts of Len and Kaito (and some possible nature), Miku's dress flipped up, giving Kaito an eye-full of her panties. Of course, being how she is, this annoyed Meiko, and she jumped in the air to kick his face in, hence her leg positioning in the air. Now, when Keito dodged Len's prank, he went forward too far, and instead got Rin, who was screwing around with Luka, and, in response, pantsed Luka.
In the end, Kaito's a badass, and Rin and Len give us some good laughs.
