The collapse was caused by a civil war, which raged from the early 90s. I tried to find stats to back up the graph but I could find nothing at the source stated on wiki (FAO) and the UN's population stats database only has 5-yearly statistics. I don't think the graph is entirely accurate, or at least statistics derived from the IMF contradict it. You have to go with the overall picture. After 1995, it seems the population recovered quite quickly, maybe due to the return of most of the 2 million refugees (no way of checking whether and how they figure in the stats).

I don't think technological progress has been high enough in the agricultural sector of Rwanda to relieve the pressure that was there before. But maybe they figured out that genocide doesn't really function as population control.