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? Plus all the usual reasons for invading, land, money, prestige, religion, local politcial pressure, boredom.....
A more interesting question is how did the goths who were running Spain at the time, and who the muslim armies displaced, end up there.