You didn't say your mother's age but age may be a factor. Older women are at increased risk for this problem. There's an increased number of these fractures in women who are postmenopausal. Osteoporosis (decreased bone density) in this age group is another important factor.
Without its normal resiliency, the simplest, everyday stress can cause damage to the bone. Anyone who has arthritis is also at increased risk. Often, the osteoporosis added to any slight knee deformity can be enough to cause this problem.
Other factors include alcohol use, Crohn's (intestinal) disease, and the use of steroids for arthritis. Low calcium absorption, vitamin D deficiency, and hormonal changes are also factors.