All dwellers are radiated - how could this happen?

I've come back to my game now after a half hour. Everything seems fine - enough food, water and electricity.

But as I selected the first person to check their health, he was radiated about the half of their life. I checked all others. EVERYONE is radiated. What could happened? I have 40 dwellers in many different rooms. Is this a bug or was there another fallout? :-D

I found the reason as it happened again. My water reserve was on the edge of minimum which looks green but was too low for my dwellers. They get radiated. The radiation faded away after getting enough water for longer time.