Answer:
Because they were treated by the Arabs as second-class Muslims, heavily taxed, and even enslaved.
Step-by-step explanation:
In North Africa, most especially Algeria, the population was majorly of Berber origin, and additionally with those who followed traditional Berber religious practices.
However, when the Arabs swept in, invaded and conquer the region in the seventh century, subjugating the Berbers, they brought with them an Islam that replaced traditional practices and Christianity.
Alas, even after many Berbers converted, they were treated by the Arabs as second-class Muslims, heavily taxed, and even enslaved.
Hence, this made the Berbers resented being ruled by the Arabs, even though they had converted to their religion.