The split between Hutus and Tutsis arose not as a result of religious or cultural differences, but economic ones. "Hutus" were people who farmed crops, while "Tutsis" were people who tended livestock. Most Rwandans were Hutus. Gradually, these class divisions became seen as ethnic designations.
Because cattle were more valuable than crops, the minority Tutsis became the local elite. By the time Belgium took over the land in 1917 from Germany (who took it in 1884), an ethnic Tutsi elite had been the ruling monarchy for quite some time.