More than half of people in big American cities lived overcrowded -- that is, >2 people in a room, INCLUDING KITCHENS! Many rented just a bed for half a day! They slept, and the other half the other person, who worked in night shift, slept on it.
In big cities, the traffic in the streets, with horse carriages riding on cobble stone, and cars, started at 6:00 and lasted till midnight. Steam locomotives made a lot of noise and smoke. That's cortisol, lower immunity, more other consequences.
And bear in mind, not everyone had electricity, not to speak of central heating. You had lots of chimneys everywhere. Not everyone had sewer, tap water and so on. I guess, a good deal of these people migrated to cities from more quiet places, and since there was no notion of harmful environment.
We tend to be surprised why modernism got so much traction, and even the best architects hated cities (e.g. Frank Lloyd Wright, who wrote something like "a city plan is a fibrosis"), but the reasons were everywhere, real and brutal.
So I'm pretty sure the reason for people being nervous, is quite physical, not "people were scared", as you may conclude from the article (although this is not explicitly stated).
[EDIT] Forgot about the social environment. When you move to a big city as an adult, without the college/university to give you social fabric, you're quite lonely. And in big cities this fabric was getting thinner with urbanization. And you're short on money, can afford only a bed, and count every cent. I think it's a more serious reason to get neurotic than times changing too rapidly.