Implant failed right after placement. Any suggestions on the root cause?

Hi, A patient had two missing teeth for several years on the upper maxillary, and I placed both implants on #13 and #14. The implant on #14 failed immediately post-surgery. The bone height on #14 was 8mm and the bone width was between 6 to 4mm I placed simultaneously the bone graft and the implant (5.8 x 7.5mm). The patient started with complications immediately post-surgery: wound dehiscence, infection, and bone loss. I cleaned the infected area, added more bone graft, and put the patient under antibiotics. Three days after cleaning the infected area, the bone loss continued to progress on #14. The implant threads were visible. I am wondering if I drilled too close to the maxillary sinus, or if it was wiser to do first a bone graft procedure. Any suggestions on the root cause?