In general, it should be acknowledged that sources are scarce and rather silent on what exactly happened. There are no contemporary Muslim sources, and only one Christian source.

Hugh Kennedy in "Muslim Spain and Portugal: A Political History of al-Andalus" makes several interesting points worth noting though:

• political and economic pressure on the Muslim troops and elites in contemporary Morocco to continue raiding and warring to gain booty to reward troops and keep internal stability

• Internal factions in Northern Africa between Berber tribes and the Berbers and Arabs

• The Visigothic kingdom being a rather weird entity, with a very small ruling class (estimated to be 1-2% of the population) with internal struggles mosly related to succession, and internal factions hoping that they could dethrone the King Roderick

• A rather strongly centralized state with a large army, and relatively weak lords

This is setting the stage for why Muslim raids on the Hispanic peninsula happened. Why did it turn into a full-scale successful invasion?

Well, there appears to have been a decisive battle where the king went missing and/or died, and the Visigothic factions could not immediately muster a response. The central army was beaten and scattered, and no clear leader appears to have emerged.

The invaders took the capital Toledo without resistance, wintered in 711/12 and were reinforced by a large army dispatched from Egypt (likely so that the governor of Egypt could be in on the booty and assert power over the newly conquered lands).

From then on, it appears that a mixture of taking holdigns by force, peaceful resolution with local nobility and perseverance led to relatively quick success within a few years. It also appears that the Visigothic antagonists of Roderick, after discovering they were not able to hold back the invasion, found some sort of agreement with the invaders. Without a strong central elements, local lords could reasonably decide that working with the Muslim invaders could be beneficial for keeping themselves and their subjects save and even preserve some power. It appears, for example, Roderick's competing faction was allowed to retain their lands.

So there you have it: A centralized state in political crisis, a decisive victory, nobles kinda wishing the king would die at the hand of the invaders, and potentially not enough military men to defend the cities - or lackluster resistance. The book I'm basing this off goes more in-depth about this all in the first dozen pages or so.

Regarding the reconquista, I'm sure others can say more, but I'll just mention here that it was the opposite as far as I know: Strong state without much internal problems, relatively scattered invaders.

So the answer then is a shattered, small ruling class, which