The reason is soccer. Last week Egypt played Algeria. Both countries are arch rivals when it comes to soccer and the tension was high already then: the bus with the Algerian team was attacked on the way from Cairo airport to the hotel en a few players were (lightly) hurt by broken glass.
Egypt beat Algeria 2 - 0. After that a play off was needed, because both countries ended wit the same amount of points in their pool. On wednesday night the two teams met again in the Sudanese capital Khartoum - on neutral ground. There Algeria beat Egypt 1-0. Thanks to a goal by defense player Antar Yahya it´s the Algerians that will go to Johannesburg in 2010. But afterwards there was more beating: Algerian fans chased Egyptians in the streets of Khartoum with knives and all sorts of weapons, Egyptian radio- and tv-reporters fled and were isolated for hours in a resataurant, the sons of president Mubarak had to take refuge in their hotel. People were wounded, one Egyptian girl reportedly lost an eye.
The next day Egypt summoned the Algerian ambassador and recalled its own ambassador from Algiers. The Egyptian football Federation condemned the behaviour of the Algerians and withdrew from the North-African Football Federation. It´s a full blown crisis.
PS. What I almost forgot: after the first match the Algerian newspaper Al Shorouq reported that Algerians had been killed in Cairo (which wasn´t true). After that angry Algerian mobs burned Egyptian flags in the streets of Algiers (see video). As you can see on the above picture the Egypians yesterday answered in kind.