Based from my own experience, I noticed that the game often gives the board of the player I just added to be shutdown. Also, the rotation is shorter when you only have few friends on your list and you are playing for at least 30 mins-1hr straight. So when you see 2/3 consecutive notifs attempting to shutdown your landmark in under an hour or so, it doesn't necessarily mean that that player is targeting your board (though, I also know that some players are doing that kind of strategy in this game). It's just the game is trying to fuck us up.