Why do women have bleeding problems years after hysterectomies?

You should not. You should not have bleeding after a hysterectomy unless you had a supracervical hysterectomy where a portion of the lower part of the uterus was left behind. If you are having bleeding you need to be examined by your doctor. A common cause is some extra scar tissue called granulation tissue at the very back of the vagina.
See your family doc. If your having consistent bleeding i would recommend seeing your family doctor or gynecologist to make sure it not a serious disease process.