Almost all of the information that I can find with regards to the Moorish invasion of the Iberian Peninsula simply states when the Moors invaded. There seems to be very little on what the motivation of the Moors was to invade the Iberian Peninsula. Is this just a little studied area of history? I'm hoping someone can provide some academic sources supporting a reason why they invaded, and not just stating that they did invade.
Actually the motivation is pretty well-known. The motivation for the invasion of Spain was similar to that of all Muslim conquest of the period. Islamic armies under the command of the "The Rightly Guided Caliphs" and the following Umayyad and Abbasid caliphs benefited from a unifying religion to form a large and motivated armed forces, out of what had been inter-warring tribes (exactly how this was managed is still difficult to fully comprehend).
Much of the initial conquest was focused on using the established tradition of caravan raiding to gain wealth and find weaknesses in neighboring states. One of the major motivating factors of this conquest was less on missionary zeal, and more on gaining wealth not just on conquest, but from the jizayh tax leveed on non-Muslims. This motivation is one of the most potent reasons as to why Muslim civilizations were less evangelical and more accepting of Jews and Christians, who they saw as fellow "people of the book", monotheists, and eligible for jizayh tax (other non-Muslims had to convert or face exile). The initial raids by mostly Berber converts to Islam where motivated by this and ultimately culminated in the Battle of Guadalete and the establishment of Umayyad rule over much of what is now Spain. This process of conquest and its motivation were similar to the process that lead second of the "Rightly Guided Caliphs" Umar ibn al-Khaṭṭāb, to capture the Sassanid empire and most of the Byzantine empire.
The key point is that the motivation to invade largely Christian and Jewish Spain was based on both the wealth from the initial conquest and the wealth generated by the jizayh tax on the population.
To learn more I would heartily recommend the following sources on Islam and Arabic history:
"because it was there".
If the Moors were going to invade anyplace under the leadership of their Arab overlords it would have to be either Spain to the north or parts of Africa to the southwest, south, and southeast - and those regions of Africa were bordered by or actually in the bare and barren Sahara desert.
Because Spain was wealthier than the Sahara, Spain was invaded first and the Sahara was conquered and/or converted to Islam slowly over centuries by various Muslim groups.s
After Muhammad conquered the entire Arabian Peninsula in the 1st Jihad converting it to Islamic rule, four Califs took up the sword of Islam, after Muhammad's death in 632 AD, to proliferate the revelations of Muhammad's Quran throughout the world. Particularly Calif Omar who began referring to himself as "Commander of The Faithful, who conquered vast masses of territory stretching from The Middle East far into Africa. In less than 100 years after the death of Muhammad, The subsequent califs conquered the Persian armies capturing Babylonia, Mesopotamia, Armenia, and Persia. To the west they conquered the Christian provinces of the eastern Mediterranean, from Syria and Palestine to Egypt, Tunisia, Algeria, and Morocco. In 711 AD they conquered the Iberian peninsula and established a strong foothold in Europe, which led to conquering Spain and Portugal. By 751 AD Islam had conquered India and Central China. In fact, in these short 100 years they spread their political / religious totalitarian ideology, ruling over of planet earth, greater than The Roman Empire at its glory. Not until Pope Urban II and the launch of The First Crusades, were the Muslims seriously challenged. But it wasn't until after the Renaissance and the industrial revolution in Europe, that Western Civilization had the technology and wealth and political might to recapture these great ancient civilizations and Christian lands from the Muslims and prevent Islamic World-wide Domination.
Western historians say they invaded seeking only loot and land. As they did elsewhere. Eastern historians say they were responding to a call to arms against the king of Spain; by a neighboring Spaish duke whose daughter was molested by the king of Spain. It was probably a combination of a Muslim Moorish penchant for conquest and Christian Visigoth disunity.