Translatory summary from the comments: Apparently the black lady was using two seats and refused to move after the Chinese lady asked nicely, at least this is the Chinese lady's version, after the fight starts some of the other Chinese women are screaming "hit her!"

And the video description:

"Oct 7, 2009 10am, Chinatown San Francisco. This fight occurred on the Muni Stockton route (the 9 i think) just before the stockton tunnel. As you can see, a simple argument about seat-hogging quickly turned into a full blown brawl. It's pretty clear who threw the first punch, you can see the bus' serial number, and the exact time in the video.

By the way, this is the same bus where I was attacked 3 weeks ago, so I thought documenting the event was the most important thing I could do. This fight was NOT reported by the driver OR by the MUNI person who happened to be at the final stop (Stockton and Sutter). I'll be walking to work for a while."