Here's my stab at it anyway.
The Umayyad Caliphate launches a small fleet west to capture the canary islands for whatever reason, prestige, an island base, to spread the word of islam, etc. This is an era of conquest, it's entirely possible some ambitious governor of Morocco might want to give it a shot.
There were jews who served in the caliphate and the navy tended to be have more opportunities for advancement than elsewhere (the maritime port of Sidon was governed by a Jew in the 10th century).
So lets say there's an expedition sent to the islands. With a small numbers of arabs, berbers, black africans and some jews (which would not be an unreasonable ethnic makeup for the time period). Then the berber revolt breaks out (maybe delayed from OTL). Word trickles to the canary islands, there's a withdrawal of troops and also ethnic strife between the berbers and the arabs.
Before long the caliphate is cut off from the atlantic by the berber states. No more ships are coming, the army attempts to withdraw entirely but their ships are either wrecked or destroyed in the ethnic fighting. They're now left on the canaries to fend for itself the way the OTL Syrian army retreated to Iberia and stayed there.
Some particularly ambitious jewish sailor rises to the top in this chaos. He makes an alliance between the remaining soldiers and local guanches, marries a guanche women and establishes himself in control of a large area of land.
As a result of his influence, Judaism or a form of Judaism becomes picked up by the other guanches and spreads among the islands. Evangelicalism is encouraged by the situation, in order to have jewish girls for him to marry. Resulting in a canary islands that is already majority jewish by the time any Iberians decide to turn up, if they ever do.