Yes, it might have been cancer. Of course, I am just considering the medical reasons for a tubal ligation and not the Catholic teaching on the matter. One other event that can make a future pregnancy dangerous is when there have been several c-sections. In some women scar tissue can build up, and there can also be a danger of the uterus rupturing if there is a future pregnancy.
Of course, we are most likely not going to know details in this case, but there are indeed medical situations that would make a pregnancy dangerous.
There are certainly medical situations that would make a pregnancy dangerous, and would therefore make the avoidance of pregnancy a moral responsibility. However there are moral ways of avoiding pregnancy which do not involve self-mutilation (directly intended sterilization.)