I get annoyed when strangers talk to me. Especially when I'm listen to music. I was returning forgotten textbooks to school on Monday, and as I was sitting at the bus-stop, an old lady told me that it was cold that day. No it wasn't, it's summer. A few months ago, I was getting off the bus, and this old guy (who seemed a little crazy) mumbled something about umbrellas to me, so I was like, "yeah..."
I think often random people will start conversation if they feel the situation is awkward.. to kinda release a bit of tension. For example, sitting at the bus stop, I think they are just being friendly and although you find it annoying we all have to be tolerant and realise they have good intentions.
Also, old people may not realise you're listening to music.. or know what headphones are!