The redrawing of territorial boundaries in Africa did not take into account the ethnic and linguistic groups there. If you look at the map of Africa, there are strikingly many straight lines - unlike in places where the bourders formed over centuries of re-negotiation (such as in Europe). The redrawn boundaries often split nations in half and grouped hostile nations together, often in similar proportions, which meant that no nation had the majority. Another case was that the minority of one country was a majority in other.
This has created a potential for civil and other wars, which unfortunately did happen and continue to happen.