The entire clause explains what sparked the fire, winds [bringing] down power lines. However it would be unusual to say the wind alone 'sparked a fire', so the downed power lines (which is capable of sparking a fire by itself) is the most proximate cause.

"Sparking a fire" is an adjective clause. It is impossible to answer your question because the sentence construction is ambiguous. The adjective clause could modify "winds" or "power lines". Both interpretations are correct, but in most cases the listener resolves the ambiguity by attaching the clause to the nearest candidate, "power lines". In reality, it doesn't matter whether it was the winds or the lines, so the ambiguity does no damage. If you want the sentence to be more clear, you can write either "Sparking a fire, the winds brought down the power lines" (the winds did it!) or "Winds brought down the power lines, causing them to spark a fire". (the lines did it!)

I think Gilgy explained it well. Also, I believe the sentence needs a comma, which might make it clearer: "Winds brought down power lines, sparking a fire."