Two days in a row on my commute home I've had vehicles behind me start honking at me. (Note: I'm in Canada, and drive on the right.) The first time I was riding on a four-lane road, with a left turning lane and a right turning lane, in the second lane in from the right (since I was going straight through). The driver behind me honked a few times, pulled up beside me less than 0.5m away (the next lane to the left was occupied, and the lanes are fairly narrow) and continued honking and started shouting at me. I waved at him to back off, but he ignored me and continued yelling.
The second time, I was on the same road but not at a point with a right-turn lane, so I was hugging the curb. Again, a vehicle behind me started honking until a gap in traffic in the next lane over, when the driver passed me (this time with appropriate clearance and without any further fuss once they were no longer behind me).
Especially in the second case, I wasn't sure what the driver was honking for, i.e., what they wanted me to do. Go faster? Get off the road entirely? What should I do to determine whether someone is honking at me for a legitimate reason or just out of impatience? And how do I respond to what, as far as I can tell, is unwarranted honking, to indicate to someone to back off a little if necessary or at least stop being dangerously distracting?