I've noticed a new pattern after what seems to be an update yesterday. (I got a pop up saying some things have changed assuming that was an update?) Anyway I run 3 vaults at a time, often spending about 5 minutes or so on one before switching to micro another. I usually spend an hour or two when I can doing this. I've noticed that about 2 minutes after every switch (or loaded vault) I get attacked. It's every single time. Try it. log in mess around, and wait about 2 or 3 minutes (as long as your vault is out of tutorial mode of course) and you will be attacked by whatever normally attacks you at your level. Then switch vaults wait about 2 or 3 minutes it'll happen again, repeat as much as you want until you decide it's not just random. I'm not sure why they added this but it's kind of annoying with my higher end Vaults. I usually spend the first few minutes checking my upgraded dwellers and moving them where they need to go and clicking on the resources etc. Unfortunately this is when the attacks happen so it's kind of annoying to be interrupted during that process.
Also side rant: Why is it necessary to click on resources to collect them? That's really annoying and distracting on bigger vaults. My rooms generate resources in under a minute so it's like every 30 seconds or so I gotta click to recover my whatever. There is no reason that this shouldn't be automatic, as it's automatic when you are away from the game anyway. /rant