Honking your horn is just a way of saying, look out, I'm here too. Weirdly enough people see it as being aggressive.
It's weird though how motorcyclists here think that having your blinker on to turn left, actually means: Wow, how kind, he'll let me pass on the left!
So many times I've experienced this. I'm just driving closer and closer to the sidewalk to make less and less space for them, sometimes I just turn left instantly. They always turn their head, looking at me as if I'm crazy, but my blinkers were on for about 20 seconds or so. Motorcyclists should maybe just get a tougher exam before they may go on the road.