The primary beef seems to be between the Bandidos and the Cossacks. The Scimitars were there, based on pics and a club called the Vaqueros. One initial report said this was to be a "summit" of five clubs.
http://www.nytimes.com/2015/05/19/us/waco-texas-biker-gang-shooting.html
I've heard a couple conflicting stories about the root cause. The Bandidos claim Texas as their own. The Cossacks were formed in Texas so there's the root conflict. I've seen one account that the Cossacks have loosely aligned themselves with HE, sworn enemies of the Bandidos.
Saw a news report today that the Cossacks have active and former military ties and are obtaining frag grenades and C4 from them for the impending show down.
8 of the 9 dead are identified as Cossacks.
Well, we can always rely on the statements of 1%er MC's......