Fallout – nywhere to submit bug reports for Fallout Shelter

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.

Try to avoid upgrading rooms. Incidents are much worse in fully upgraded rooms than a room with hasn't been upgraded. In particular training rooms don't benefit much from upgrades so I don't bother to upgrade those rooms. I also don't upgrade medical and science rooms, so I can rush them over and over as necessary to generate stimpacks/radaways or to get rushing or incident related objectives. Most of the time I leave these later rooms occupied by a single dweller, and that's enough to deal with any incidents.

The only rooms I do upgrade are the energy/water/food resource production rooms. For example I have fully upgraded a Nuka-Cola bottling plant next to the vault door, and the dwellers there are armed to the teeth. I also make sure that any production room that I have upgraded is fully staffed. I don't bother to try to respond to incidents by moving people around. It never works well. (Though early in the game I did move weapons around. One missile launcher can make a big difference.)

You should also consider your vault layout. Storage and residential rooms should be left unoccupied and placed so incidents in them can't spread to other rooms, or at the very least only to one fully occupied room.

I've also limited the population of my vault to about 110, as there doesn't seem to be much point in going beyond that. It's already pain to deal that many dwellers. I'm not sure if vault population factors into the severity of incidents, but that might be something to consider. I don't seem to have as much of a problem with incidents as you do even in my fully upgraded rooms, but that could be because my dwellers have better weapons than yours. I'm currently tossing away anything that doesn't do least 10 damage on average. At the very least reducing the population of your vault would let increase the damage of the weapons the remaining dwellers are equipped with.

Finally, I'd recommend just putting your phone in your pocket when you can't pay close enough attention to respond to incidents. Unless you're actively grinding certain objectives, you might as well just leave the game alone. You can't have any incidents while you're not playing the game.