I think it's the number of people they have to save in such a short time. Using the firefighter example, it would be like all of a sudden, the whole city caught on fire. And then once the city is out, the next city catches on fire and because there aren't enough firefighters there, you go to help. And then once that city is out, the next, etc.
Added on top of the fact they are worried they'll catch the virus and maybe even spread it to their loved ones. In the US at least, I imagine most nurses thought they would work with less or non-infectious diseases, like cardiovascular diseases, not an outbreak that could lead them to infecting and killing people they loved.
Again, I think there are just some variables that make this situation different than what they thought they were signing up for.
