Magic came to North America with the arrival of the dragons. As they insinuated themselves into the power structure of Xiyatu — the city is still officially Communist, but all members of the Central Committee are serfs of the dragons — they began forming schools of shadow warriors and magus traditions.
Dragons are not a mage race, even Great Dragons — they cannot develop all of the five mage talents. However, they are innately magical, and they can (through their serf bond) help humans develop these talents, and those humans can teach others. Though the way this happened seems out of character for dragons.
You’d expect dragons, in a new world, surrounded by new sapient races, to enserf them and train them. Indeed, the entire conquest of Xiyatu is prototypically draconic: they took the city, increasing their stature, and trained shadow warriors, sorcerers, and maguses, which increased their stature but also secured the clutch against outside threats (Emerged and otherwise). The initial confrontations with humans turned violent, perhaps inevitably, and by taking Xiyatu they neutralized that threat.
Yet, after taking the city and developing the magical talents of their serfs, they didn’t act to compartmentalize that information. You’d expect dragons to hoard information the same way they hoard eggs or wealth. Mastery is stature. And they do.
Yet they took no care to ensure that magic remained a secret only of their serfs, or their city. And so magic spread.
Even before Beyonders began to Emerge, shadow warriors trained in Xiyatu were roaming through the Outlaw, taking on acolytes and spreading their disciplines. Maguses began training others, establishing their traditions across the continent. Artificers, sorcerers, and shadow walkers all began to teach others. Beyonders added greatly to the knowledge base of all talents, but they were already known before the Beyonders came.
The power of magic was flooding the land, because of the vortexes, and magicians were spreading rapidly, because of the dragons. Magic in the Outlaw sprang from the dragons of Xiyatu.