What was Patches doing? Sleeping? Close to where Zoey fell of the bed? Sounds like Patches was startled by the fall and maybe thought Zoey was coming after her. It's just something outside of the usual. Sometimes it's enough to set them off.
You did the right thing. If they start again then separate them for a longer period. If it keeps happening you may want to consider taking them both in for a check up (separately) and then doing a re introduction to each other.
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!