Although there had always been class divisions, the colonial Belgian government decided in the 1930's to officially categorize the Hutu and Tutsi. They tried unsuccessfully to draw artificial distinctions between the two, according to Antoine Rutisiyisire. You ended up having one brother being a Hutu, because he doesn't have cows, and another one becoming a Tutsi because he has cows. We still have those things around here even today. But I think it was kind of an administrative mistake.
Many historians are far more critical.
They say the creation of a Tutsi elite, which enjoyed superior education and job privileges, bred resentment among the majority Hutu. A civil war began in the early 90's, and when Rwanda's Hutu president died in a suspicious plane crash in 1994, there was an all-out call for blood. Killers were allowed to take their victims' belongings, and victims were easy to spot. So they look at your identity card and they find you are Hutu or Tutsi. It was that simple.