I have only been true dispersed camping a couple of times but regularly go to far back off the road off to ourselves. Some folks just like to be close to someone else for security reasons or whatever. It always happens when you get setup way back somewhere someone else will follow suit and setup close to you.
We like our privacy but some of these way far away places it does give you a good secure feeling to see someone else once in a while I guess... I too would not be pleased to have them right next to us... I would most likely move away from them...
I even see this at the Walmart parking lots. One camper will setup and the next morning you will five or six all grouped together haha... Even thou there is plenty of available space for overnite parking...
Human nature I reckon...
Roy Ken