The Dayaks are in a panic, they are feeling marginalized. They have been provoked for many years. The Madurese are violent. They fight in the markets and in the farms. We don't have similar problems with the Buginese or Chinese or Javanese.
Economic rivalry is a factor.
Many of the Madurese competing for these lower rungs of the economy were recent immigrants, fleeing the poverty of their native Madura, desperate for work. At the site of the largest massacre, Parenggean, the town's main industry was controlled by the Madurese loggers. To make matters worse, a Forestry Department official says, the Madurese had persisted in logging forest that was sacred to the Dayaks. Now the sawmills in the town are quiet and rows of huge logs lie abandoned. These economic tensions, added to the age-old stereotypes of the Madurese as clannish, threatening and rude, made it easy to roil the Dayaks.