Fallout – All dwellers are radiated – how could this happen

I had the same problem as well, 70% of my people died when I closed my ipad and went out for about 15 mins. From the battery consumption I think the game didnt go into sleep mode, or it prevented the ipad to enter sleep mode, so the vault ran out of resources and dwellers died, etc. It was a vicious cycle trying to get back to normal. It took me two hours to raise the deads and stabilise my vault because the game threw all kind of catastrophes at me when I'm low on everything. Now my main power station still has a "something died in here" debuff that makes people terribly unhappy in that room. I think we need to log a bug to bethesda.

The only thing which could explain it (besides a bug) is that you may missing some electricity or water. If you haven't enough electricity your food and water supply will fall down and maybe collapse. This will increase the radiation of your dwellers. In some cases they may die on this.

Otherwise, I would say it could be a bug. I've had a situation which was nearly the same. In my case electricity was too low, which leads to the situation I described above.