Cat fight after accidently stepping on my cat

We have two cats. Recently my partner accidently stepped on one of the cats. In an instant the other cat really aggressively attacked the cat that was stepped on. Almost as if he spotted a moment of weekness and attacked him. They never fight normally except when this happens.
What is the cause of this.? What can be done to stop this? Thank you

It's actually normal behaviour for cats that are in pain, they strike out and your other cat was the unfortunate target. 

Here's an article on re-directed aggression.

Happens to me sometimes too. A woman in line dropped a jug of milk on her toe and I immediately wanted to scratch her.. lol. Joking aside, I've had to seperate cats for that very reason and reintroduce them over a period of weeks. Redirected aggression sucks.