All it takes is some small event to cause a chain reaction. Cats are sensitive to you and each other. Can be funny at times.
A few weeks ago one of my cats came running into the kitchen at a high rate of speed, slipping on the floor, causing all kinds of noise. (This was in play) Which scared a couple of them laying on the floor. Everyone got scared. Bumping and hissing at each other. So the word got out that their is a monster in the house, and its going to get everyone of them. In the end all five where freaking out. Taking it out on each other..... Calmed down after awhile. lol
Don't separate them. You are only feeding the fear. They will sort it out :)