What causes pain a year after the extraction/bone graft?

it is not normal to have pain a year after extraction and bone graft You could see your dentist.  The pain might be coming from  teeth adjacent to the site which could be diagnosed by exam and xray Your dentist will refer you back to the otal surgeon if needed  

i think this pain is not from that extraction and grafting. its my advice to visit dentist . have to check any king of infection present in neighbouring teeth.

It looks like the pain had rather a sudden onset in the area that has healed since the previous regenerative surgery. My first suspicion would be another tooth adjacent to the area where the graft procedure was performed. See your dentist, such things can be easily diagnosed. Good luck !!