Post Implant Infection: What are potential causes?

Hello – I had bone grafting, dental implats and crowns done about 5 years 2006/2007 ago on teeth #9 & #10 upper jaw. It has been a great experience and I have been 99.9% happy with the results. About 2 mos ago I noticed that my gums above the implants looked swollen and irritated and eventually noticed infection/pus coming from the top of the crowns – I believe #10 is the culprit. Went back to the oral surgeon and he put me on AMOX CLAV for 10 days – infection and gum irritation has “almost” cleared up and he reports no bone loss and implants are tight and am scheduled for follow up. My question is what could have caused this infection after so many years? I am diligent about brushing, flossing regular dental check ups … outside of something such as getting bumped near the mouth by my horse and my oral b electric toothbrush I am at a loss as to why this would happen so many years later. Please advise and thank you for any guidance!