Here's a weird story.

So I got a quest to clear out raiders from the Corvega Factory from a settler at Ten Pines bluff (extreme northern edge of the map). I have a few quests lined up and I take care of another one first. That one leaves me near the Corvega Factory so I head there by foot.

On the way I see a fight going on with some raiders. Never one to let raiders die by someone else's hand if I can help it, I join the frey taking on the raiders. That's when I notice that who the raiders are shooting at is a 'Settler' who has been outfitted by me as one of my early guards (light combat armor, a hard hat and a decent weapon).

I have no idea why that guard is walking the wasteland, and there is no conversation option other than trade. I give the guard a few stim packs and let them continue their mysterious journey. I loot the bodies of the raiders, head to the factory, see some supermutants in the distance and engage them for XP (I'm a sniper). I notice that my settler is headed straight towards them as I do, and pick off a suicider as it charges him. The mutants dead, the settler continues walking south.

So I get busy clearing the factory. It takes a little while as there are many winding corridors to wander. The job done, I head back to Ten Pines Bluff to report the deed. The quest giver is nowhere to be found. I check my map and the quest indicator shows my quest point to be just above Somerville Place (a settlement I have built up) at the extreme southern edge of the map.

I go to Somerville Place and check my map again. The quest marker is now 'at' Somerville place. I look around and sure enough, there is the very same settler I found wandering the wastelands, standing guard in a guard post in Somerville Place! She thanks me and I get the quest reward for helping Ten Pines Bluff.

I did NOT send any settlers from Ten Pines Bluff to Somerville Place. This was something the game did on it's own, and they walked the distance.

Is this sort of thing a normal game behavior that I have only just now noticed? Do settlers migrate of their own volition? Is there any reason for this that we can anticipate and plan for?