Although no cause for the blocking of YouTube by service provider Maroc Telecom has been ascertained, the current speculation is, as Moonlight mentions, is:

La raison de cette censure serait la publication par ce site de 2 vidéos d'un islmaiste malade mental qui (en cachant bien son visage bien sur) a parlé de la fortune du roi en l'insultant.

Maghrebism is encouraging readers to sign a petition for Maroc Telecom and parent company Vivendi to restore YouTube access:

The Moroccan government is usually very sensitive to media-attention. They are often very afraid that their image of a moderate, safe and open muslim country will be damaged.
Not realizing that they damage that image themselves.
So please sign and digg away because that is the only way to make them realize that what they did is wrong.

The petition can be found here.