How old is this fan?
It could have been dust in between the rotor and the stator.

I would not leave this fan running unattended

It isn't a ceiling fan, its a light fixture in the ceiling. Would your answer still apply?

My guess is that there was a short that developed over time, possibly caused by cracked insulation on a wire or a loose wire nut. The short caused a little bit of the wire to literally burn away, so that now there is no more short - at least for awhile.
Whatever it is, you should get it checked out. Next time it could start a fire.
You didn't say if the flash was inside the fixture. That is worth knowing if you noticed which it was. The problem could be with the light fixture, or it could be in the house wiring.

Gezzz .. I don't know where I got a "ceiling fan" fan from, must have been a job I did that day ....
Anyway, it could be rpxlpx said or it could be that a strand of the "hot conductor" did not make it under the wirenut completely. I do recommend that you take down that fixture and inspect the connections and the integrity of all the conductors.

Any chance the fixture has several lamps in it? May have been one blowing out.