This attack was actually by a Pied Currawong, not a magpie.
I was cycling a short distance through the city on the footpath, not wearing a helmet.
The magpie aggressively swooped me 4 times, making heavy contact with my head, It perused me for a long distance through the city's pedestrian mall: 2 blocks.
Not sure what set it off, as I've not been swooped by a Currawong before. My lack of bike helmet may have given it more confidence.