Why are they fighting?

This morning Roxy and Villy were in the back garden, by the back door, when I could hear one of them yowling at the other. I went out and they were both puffed up, and Roxy was yowling at Villy. DH went to pick Roxy up but she snapped at him and ran away. I brought Villy in and made sure she was ok and then I went back to see Roxy, who had calmed down and then I brought her in too. The girls still seemed to be wary of each other, Roxy puffed up her tail again but they didn't fight or anything. They are acting almost normally now. What's up with them? Did they have a disagreement? They are both speyed.
Jess

p.s. Roxy was sniffing a lot when she came in. They sniffed each other too and then layed down near each other.

Might have been something else that scared them. I'd sprinkle both with cornstarch baby powder and rub in the fur.

They almost certainly saw or smelled another cat.

Thanks, there are a lot of cats in the neighbourhood so that was probably it. They seem to be back to normal now, and they went out in the garden again today.