The wave of Islam that spread across North Africa was quick and arguably welcomed but why did they just keep going and going to places it was clear they were not welcomed? Why Europe?
What were they trying to accomplish and how far were they planning to go?
There isn't much in the way of motive, with regards to the North-African invasion and subsequent conquer of the Iberian Peninsula, though a few theories have been posited.
North African raids against Europe were common both before and after Islam, and the large raiding party that attacked the Visigoths in 711 resulted in such overwhelming (and possibly unexpected) success, that what was originally another raid turned into a full-scale invasion.
King Roderic's (Visigoth King) rise to the throne/rule was controversial, and may have resulted in disillusioned nobles fleeing to North Africa and asking for help/intervention. The Berbers responded, and after easily routing Visigoth forces, decided to turn a quick intervention into a permanent occupation.
I believe that attacks against the French, while romanticized in the West as epic struggles, were little more than occasional and ordinary raids.
Muslim forces didn't seem to have much interest in actually invading France, though if their leadership had never fractured internally, and they had been able to retain the Iberian peninsula, they likely would have attempted it.