When chickens first meet each other, they have to sort out the pecking order (who is allowed to peck who, and which one has to move out of the way.)Oh ok, good to know!
They do this by chasing each other around, ruffling up their feathers to look bigger, staring at each other, and sometimes by actual fighting.
All of this is distracting to a hen that is supposed to be sitting on eggs, and eggs or chicks can get stepped on and squished.
Once the chicks have hatched and are a fews days or a week old, they will be able to run out of the way if they need to.
After the chicks hatch, it would be good if you can arrange to have the other chickens in a pen next to where the mother & chicks are. If they live side-by-side for a week or two, they can all start to know each other through the wire (ruffle up and stare, but no biting or chasing), and will probably settle things much faster and more gently when you actually put them together.