Due to some people from Outlawz who suicided on Gurentai, Gurentai decided to kill Outlawz rather than accept compensationmoney.
Rossineri, Gurentai's blood, helped them with that, while Riot, Outlawz' blood, jumped in to defend Outlawz.

There is a big discussion between Trice (top of Gurentai) and various tops of Outlawz and Riot about who actually started this war, both claiming that the other side started it. Read the comments yourself to form a clear opinion.