I imagine she hasn't divided them for years. What happens is they get pollinated, she doesn't remove the seed pods, the pods pop open and spill all among her hybrid iris. The iris that come up are the strongest toughest throw backs to a natural iris color, either yellow or sometimes lavender/blue or white. The only other way this could happen is from a chemical spray of some sort. Chemical sprays usually deform the flowers as well as mess with the colors, but less likely to turn them all the same yellow. To keep your hybrid iris, you must remove the seed pods or ovary which are under the flower. It's the thick part above the stem but under the flower. I had a lady in my garden who insisted that hers had changed over the years and she always removed the seed pods. She was so adamant that I finally asked her to show me on one of my blooming plants how she removed it. She carefully took off the flower petals and left a very pregnant seed pod on the stem. Carolyn in Los Gatos, CA