Hope your doctor would have given Long knee brace to protect your knee. So improper posture with the knee brace on, very less chance of graft injury. Improper(overaggressive exercise), deep squatting,running, slipping and fall again can cause graft rupture.
Next Steps
slow progressive exercises to improve knee bending,strenghtening,balance is required as per your doctor/physio advice
Health Tips
your graft takes 8-10 months for ligamentisation. so you need to be careful till that.