What Causes Ovarian Cyst With History Of Tubal Ligation?

I have a cyst on my right ovary......I had a tubuligation at age 29 after my third child. At age 40, Doctor did a partial hysterectomy removing one ovary due to a large fibroid tumor on the outer part of my uterous. when confronting about this cyst, Doctor said an egg must have released during ovulation; but I was always to understand I would not see any more eggs released due to the tubal I had.
So why would I have a cyst now?