What Causes Breaking Of Screw Inserted At Knee For Femur Fracture?

I broke femur 8 months ago, four months ago had one of two screws at knee removed due to possible infection and now just recently compressed the fracture and remaining screw at knee broke. My Dr. Says it happens all the time and now that bone compressed healing may be easier and no removal of or putting in replacement screws. Wondering how a screw can break like that and if it is starting over?