Three things I have learned so far reading Richard Fletcher's Moorish Spain:
1. The "Arab" conquest of Spain was mostly accomplished by Berbers.
2. Transportation in the ancient world between Morocco and Southern Spain was easier than within Spain, thus facilitating the conquest.
3. There was a significant population of Jews in Spain in the eighth century who were badly treated by the Visigothic regime that had succeeded the Roman Empire and who most likely welcomed the comparatively more tolerant Muslims.