… your parents or some ancestor was a wizard. It’s often called genetic magic, but since it doesn’t usually work like genetics at all, let’s call it hereditary magic rather.
… you were born when the moon was in the 8th House of Aquarius. Or some other astrological occurence. What happened with other people who share your birth date?
… you have lived in a certain place for long enough. Unlike astrological magic origins, you do not necessarily have to be born there. Unless you’re a hermit, other people will probably have magic too. And what happens when you leave?
… you performed some deed. This might be a one of a kind thing or staple of your culture. Hunting certain beasts, climbing mountains or towers, kill the demon lord.
… you found an artifact. Or several of them. Who made this thing? Why were they wizards? Are there similar things?
… you made a deal. Usually called a pact. Or again several of them. What’s your end of the bargain? What kind of entities make these pacts? Why?
… you had a very, very bad day and – boom – the magic happend. Such triggered outbreaks do not necessarily entail you wearing a cape. Oh, and try to be nice to people. Otherwise they might – boom – magic.
… you got medicinal treatments. Who performed them? Who else got those too? What are the side effects?
… you learned how. Where can one learn? What learning resources are there? If it’s institutionalized, who owns the institution? Why have they set it up? What are the requirements to get in?
Am I missing something? Probably. And of course you several of these might come into play together.