Are they indoor only cats? Maybe they need some grass. You can grow pots of cat-grass for them. Cats need fibre too, even if just to help with furballs.
![sick :sick:](https://www.purrsinourhearts.co.uk/Smileys/purrs smilies/sick.gif)
Or perhaps something in the salad smells like catnip? Or are they really after what you put on the salad, assuming that you're not veggie?