I notice that pinning behaviours also seem to occur by topic. People go out searching for a topic like cabins and as a viewer, one will often see a batch they have discovered. One of my own favourite boards is now called My Junk Drawer. It contains pins which are, for the most part, not my own. At the moment it sports a few things I would like, but mostly things that I've already owned. It is nostalgia.
For example, at the time I was about to become a teacher, Texas Instruments came out with its first Speak and Spell. I suppose it wouldn't seem very dramatic today, but in the early 1980's it was the first machine we could own that could speak without using a tape. I hadn't thought about it in years. Pretty cool.