Have you ever heard of a cabbage producing multiple heads?
Certified GKH Gardening Expert
I have heard of a cabbage plant producing second heads after the first head was harvested. If your cabbage simultaneously produced many heads on one plant that is an interesting discovery! You could try saving seeds from the plant if it prudices any.
If you didn't grow this plant from purchased seeds this year, it's possible that it is a cross between ordinary cabbage and a brussels sprout plant, since these are two varieties of the same species and can cross-pollinate.