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I can only offer one solution, and that is because iris color is, solely, a genetic trait. The only thing that I could think of that would have caused this is outcrossing, or hybridizing with any nearby beds of colored iris. They can drop seed, and germinate, which would cause the iris colors to take the trait of both, or either of the parents.

These articles will help you.

https://www.gardeningknowhow.com/ornamental/bulbs/iris/growing-siberian-iris.htm
https://www.gardeningknowhow.com/ornamental/bulbs/iris/growing-flag-iris-plants.htm

Most iris do not rebloom although breeders are working on that. They do benefit from being divided as the rhizomes become very crowded otherwise. Being crowded stresses the plants and may result in fewer blooms. So division and replanting is beneficial. It's much easier to do this if you cut the large leaves by half. Work some compost into the soil. The "mother" tuber/rhizome is the one that bloomed this year. It will not bloom again so should be removed. Offshoots from the mother grow to bloom in one to two years. Here is an article with more information. https://www.gardeningknowhow.com/ornamental/bulbs/iris/divide-bearded-irises.htm

These are the seed pods. You can try saving and planting seeds or remove them to keep your plants nice and healthy.
About two months after pollination, the seed pod will turn brown and split open. Harvest the golden brown seeds in a small paper cup, and allow them to dry out for a couple more months. Keep the cup in a cool, dry place. Be sure to keep the record of the parents with the seed.

In late October plant the seed about 3/4 inch deep and 1 inch apart in well drained soil. Again, label your plantings clearly so that you can identify the sprouts next spring. The planted seed should spend the winter outdoors, planted in full sun.

To keep the seedpod, let the seedpods ripen and turn fully brown and begin to split open before you cut them off and store them in a cool, dry place.
In the fall, as it begins to cool, plant the seeds in the garden in an area prepared with amended soil and that is weed free. Plant the seeds about ½” to ¾” deep and a few inches apart, and mark the area.
They will not flower the first spring, but they should the second. The new Iris will probably not be the same color as the mother plant was, as most Irises are hybrids and do not breed true.

This could be a new offshoot that will bloom next year, or they could be seedlings that will bloom in a few years. Either way, they will bloom eventually. It is not necessary to do anything with these, as this is a natural process.