Before my VPS folded it was mostly bot traffic, which I hadn't been expecting. Evidently many follow the HN RSS feeds and hit new posts (repeatedly..) when they show up.
Lots of iPhone users? Or all from the same address? Cos that'd be odd.
So if 10 people would load the app, their backend would make 10 requests to the icon on my server and then would relay those images to each user.
I could see something like this happening with a "push" system so when something new is posted on HN some overly clever dev decided to push the icon and title to each device to be read later but forgot to cache the icon so they end up slamming the server.