For me it seems no apparent reason. My ride home yesterday I had a perfect example. I'm riding with traffic around 4:30 pm and I'm staying to the right and people are passing me fairly normally when along comes a woman in the opposite direction who just blows her horn as she passes. As far as I remember no one had crossed the double yellow line to get around me - sometimes people do this when there is no oncoming traffic - but I have no recollection of anyone doing anythign that warranted her to blow her horn. Maybe she got scared when she saw me and just blows her horn whenever she see anything other than a car. Not sure i look like a dog cat or skunk.