I saved a game once just before a notification occurred.
It was Spectacle Island.
Out of curiosity I kept reloading the game and each time the attack occurred the foes were different.
Mutants or deathclaw or raiders or gunners attacked so perhaps it's a lottery who the attackers will be.
As far as more prone to attack.
Well in my game there are some that have never been attacked.
The rest get attacked on a rotational basis, usually two notifications in close succession.
Somerville, Jamaica, Croup, the Lighthouse, Coastal cottage get attacked most.
The drive-in, warwick Farm, abernathy, red rocket, spectacle and the others get regular attention.
The outpost and sunshine tidings never attacked except one time when i bought a fourth cow for Sunshine and the game decided they wanted to kill the cow just before delivery.
I was walking the cow home and all of a sudden the cow was transported way in front of me, to the other side of the nearby bridge with a magically spawned deathclaw closing in on it.
I had to sprint and shoot the Deathclaw before it swiped the cow with a fraction of a second to spare.
The game likes to do things like that.
Another time i bought a cow and it walked 10 paces before some mutants gave it the chop.
Murkwater gets attacked a bit.
One time the game dropped 2 high level sentry bots but one got bogged.
i think with somerville one spawn point seems to be right next to the building so my own defences do a bit of collateral damage.
Egret gets the occasional bridge attack but there is a close by random spawn point that can attack the provisioners and follow them to the settlement.
On the other side there is always radstag setting off the alarms.