This video was put together by some unknown Japanese hero a year ago, but it's making the rounds today (I think it popped up first on Fangraphs last week). And that's 100 percent of the background on the video you're going to get.
The home run bat flip is apparently an art in Japan, ranging from Random Japanese Guy No. 3's skyward hurl, to Random Japanese Guy No. 17's end-over-end forehand, to Random Japanese Guy No. 35's delicate drop.
It's interesting that this is acceptable in Japan, where scores of Western movies have taught me that they're big on honor and subtlety. In MLB, these bat flips would be considered showing up the pitcher and would lead to brushbacks the next time up. Oh shit: Vicente Padilla just signed with a Japanese team. This could get bloody.