"in 612 C.E., the Council of Gundemar of Toledo ordered that all Jews submit to baptism within the year, or undergo “scourging, mutilation, banishment, and confiscation of goods” (Meyrick 170)."
Seems like a logical reason to support the other guy.
If they wanted to support the other guy, they should have supported the Catholics of the Roman Empire to come in and overthrow the Visigothic heretics (as Rome under Justinian had done in Italy and Africa, and attempted to do in Spain). Spain at that time was ruled by Arian (not Aryan) Visigoths. By betraying all the inhabitants of the country, both the guilty opressive heretic Gothic rulers and the innocent Roman-Spanish commoners to the Muslims, they sealed their own fate in the views of both groups.