Are you actually a member of the Minutemen? "Your settlement has been attacked" messages seem to rely upon actually being a Minuteman, oddly enough.
This is only based on my own playthroughs however--I've got one active Minuteman General playthrough and one playthrough where I've done almost nothing with the Minutemen (I've only met them so I could get certain loot where they were hiding and while I nicely killed the bad guys for them I then didn't return to Preston to close out the quest).
In my NON-Minuteman playthrough, I've got a handful of settlements that you can clear without Minutemen membership. They are only attacked when I actually show up (a random possibility) and I never get any "your settlement is under attack" messages or radiant quests to help them.
Whereas in my Minutemen game, the needy bastards are constantly being attacked and generate radiant quests, regardless of high defenses, low resources, or any other factors. Generally specifically when I am in the middle of trying to blow up [insert major faction name here], stuck 400 feet underground, looting the Glowing Sea, etc.
My personal conclusion is that settlement attacks have nothing to do with what your settlements actually have--it's just that the Commonwealth really hates Preston Garvey. He probably kept trying to give them quests they didn't want.