This happens in the true boondocking, dispersed arena!! Have had persons drive into a campsite that we have "occupied" to leave their camper for the next weekend or whatever. When refused, they act like the site is their private, not to be used by anybody else, campsite. The first come idea sometimes goes out the window.
Another trick we see in the local Nat Forest sites is someone hauling their rig up a week (and sometimes more!!) early and leaving till the big weekends. Again, against NF rules, and not enforced around here. Many are left for more than the 14 days that rules state, and are only occupied some weekends.
I'll also say that these "campers" also some of the worst for trash left around the site and in the fire pits.
Leaving tomorrow morning for a site that hopefully will be available and won't have the usual drop in's after we set up!! Maybe the snow predicted will slow down or discourage some of them!! If taken, there many other sites to choose from, which is why I wonder why they feel they can squat next to you!!