I remember being taught in U.S. high school that it was deep-seated hatred between Hutus and Tutsis, but when I attended a talk given by Paul Rusesabegina (inspiration for the movie "Hotel Rwanda") in college, he said very few people cared about whether they were Hutu or Tutsi before the assassination of Juvenal Habyarimana and that the largely Hutu army simply wanted more power. I also remember hearing at some point about propaganda broadcasts telling the groups to kill each other, and I just read on Wikipedia that Jared Diamond, author of "Guns, Germs and Steel", theorizes that it was population pressure which led to genocide. So, 20 years down the road- is there a concensus?
Rusesabegina isn't totally right in his implication that ethnic tensions weren't particularly bad before the genocide. Tensions had repeatedly flared into genocide in both Rwanda (particularly 1959-1961) and Burundi (1972, 1993).
I think people forget that the genocide happened in the context of a civil war, or rather in the context of a very uneasy peace deal. The 1959-61 "Rwandan Revolution" overthrew the Tutsi monarchy and replaced it with a Hutu-dominated republic. The Revolution was typified by mass-killings of Tutsis, and led some 130k to flee to neighboring countries. Those Tutsi exiles repeatedly attempted to violently overthrow the Rwandan government, which often retaliated with reprisals against Tutsi civilians.
One of those armed Tutsi groups was the Rwandan Patriotic Front, which was based in Uganda and received massive assistance from them. It was led by Paul Kagame, who is now the President of Rwanda, who along with most of the RPF, served in the Ugandan military until the RPF invaded Rwanda in 1990. The Rwandan Civil War bogged down into a stalemated insurgency until the Arusha Accords were signed in August 1993. These accords created a transitional government that included the RPF, but were strongly opposed by President Habyarimana, who opposed sharing power with the rebels.
The civil war that preceded the genocide (and reignited after the genocide began) raised ethnic tensions to a level they had never been before. Habyarimana's government repeatedly accused the RPF of attempting to reassert Tutsi hegemony over the Hutu, and much of the Hutu population radicalized accordingly (producing the Hutu Power movement). Habyarimana was very close to the Rwandan military, which wasn't particularly pleased with the idea that they might have to take orders from the new transitional government that included rebels that they had been fighting for years. Backed by the army, Habyarimana had consistently accused all Tutsis of being a fifth column for the RPF.
In short, the tensions that exploded into genocide in 1994 had been around for generations. The civil war magnified those tensions to previously unheard of levels, with many powerful figures in Rwanda, including the President, essentially advocating genocide or some sort of mass atrocity against the Tutsi throughout the civil war.
That being said, could anyone else tell me why the Hutu Rwandan army collapsed so quickly during the genocide, allowing the RPF to take control of the country? The Rwandan military had fought the RPF to a standstill before the genocide, so I've always been curious as to why it collapsed so quickly after the genocide reignited the civil war.
Yes.
Even when the Rwanda conflict was emerging, it was already understood why. Pre-existing ethnic conflict under a single state apparatus is a recipe for "Fifth Column" politics. When the stronger party foresees its power weakening in the future, or as becoming vulnerable, and believes it may be undermined by a rising minority, they will take measures to ensure their continued survival and ability to thrive. This is not surprising - we can see this in non-violent mechanisms such as the rise of gerrymandering when the dominant party predicts less support in the future.
In states with highly-contested balances of power, the poster child being Yugoslavia 1990, we can predict conflict (peaceable or violent). The threat of physical violence from pre-existing enmities spurs the stronger party to use violence, justifying its use as a preventative measure. In other words, "We gotta get them before they get us!"
This was discussed in the hallways throughout the 90s but the dominance of old "Cold War" fogeys in academia meant that ethnic cleansing, civil war, and Fifth Column conflict did not emerge as a serious, tenure-worthy study till after 9/11. Anyway, the gist is that the dominant ethnic tribe foresaw a future vulnerability with their livelihood at stake. To them, the threat justified the use of force. Really your first scholarship you wanna focus on starts in 2000 onward. Duffy-Toft is probably the most notable champion and critic of this explanation. Here's some literature to get you started in chronological order:
Walling, Carrie. "The history and politics of ethnic cleansing". The International Journal of Human Rights. Vol. 4, Is. 3-4. 2000.
Rose, William. "The Security Dilemma and Ethnic Cleansing: Some Hypothesis". Security Studies. Vol. 9, No. 4. Summer, 2000.
Duffy-Toft, Monica. Securing the Peace: The Durable Settlement of Civil War. Princeton University Press. 2009.
Much more contentious an issue, by the way, is 'Knowing what we know from Rwanda, what shall we do to ensure future peace?' Aside from the ever notorious but brilliant Luttwak (who basically says let them kill each other), there are a number of theories, but this goes beyond the scope of your question.