Shariah law was a way of uniting Muslims so they would all stand united under one system of law. There wouldn't be any difference in laws that existed in Iraq or in Spain. Between about 800 and 900 the main trends of thought on legal matters hardened into schools or rather rites -- the latter word is preferable when referring to in practice rather than in theory. Some of these rites, such as the Zahirite which had a notable exponent in Spain, died out after a time. Among the Sunnites, or main body of Muslims, four rites came to be recognized as permissible variants -- the Hanafite, the Malikite, the Shafite and the Hanbalite. So far as al-Andalus is concerned the only one of these which is important is the Malikite, which derives its name from Malik ibn-Anas (d. 795), who belonged to the school of Medina."

For those who do not know much about Muslim schools of thought, this development mean seem insignificant but actually this must be viewed as one of the most important first developments in jurisprudence of Spain after Muslim conquest. Malikite system of jurisprudence refers to the understanding of Prophet Mohammad's teachings and Quranic teachings as were given by a well respect jurist scholar Malik ibn Anas. Arabs all over the world to this day mostly follow either the Malikite or Shafite system of jurisprudence while Muslims in sub-continents follow Hanafite system. There is no difference of opinion among all the jurists and it is only a way certain religious rituals are carried out and practiced. There is no difference which makes the different systems acceptable all over the Muslim world. Before that, Spanish Muslims were following the teaching system of a Syrian jurist, al-Awza, and about around 800, they all shifted to the Malikite system.

Christians and Jews were treated with respect and tolerance during the Islamic rule. Since they were people of the book, their rights were duly protected. Muslims had turned all their attention to the development of a unique civilization,...

The small uprisings had always been there but the main disintegration began in the 19th century due to constant power struggles. One author puts it in these words:
The disorder and civil war which may almost be said to have been chronic in Spain during the Arab dominion were due to the fact that three distinct races settled in that country were striving for the mastery, each of these races being itself divided into two bitterly hostile factions. The Arabs were split up into the two factions of Yemenite or Beladi Arabs...The Berbers, who looked upon themselves as the real conquerors of Spain, and whose numbers were subsequently reinforced by fresh immigrations, were composed of two hostile tribes of Botar and Beranis. Thirdly, there were the Spaniards, part Christian, part Mohammedan; the latter being either renegades themselves or the descendants of renegades."

These factions had their own interests in dominating territories in Muslim Spain. Their uprisings caused a serious dent in Muslim rule in Spain and signs of disintegration became obvious. By the end of 14th century, most of the territories in Spain were ruled independently which was leading to serious problems in united Spain. Consistent Christian attacks after that resulted in fall of many cities to Christians and this finally culminated in complete collapse of Muslim rule in Spain.

References

W. Montgomery Watt, a History of Islamic Spain (Edinburgh: Edinburgh University Press, 1965)

Charles Reginald Haines. Christianity and Islam in Spain (756-1031) LONDON

KEGAN PAUL, TRENCH & CO., PATERNOSTER SQUARE 1889

Watt p.1

Watt p. 5

Watt p. 7

Watt p. 63

Haines

Watt p. 82