Alveolar Crestal Fistula 2 Months post placement

64 y.o female who had 2 fixtures placed 4/11/23 into area 2 and 3. The fixtures were guided- Implant Direct Legacy3 implants. #2 was 5.2x8mm and #3 was 4.2x10mm approx 2mm of bone surrounding the fixtures. All drilling was done at approx 400rpm. The fixtures were torqued by hand to 25 newtons. Approx 2 months later 6/6/23 a fistula appeared over #3 (see photo A). A midcrestal incision was made and the area were opened. No pus was noted but the fistula was sent for C&S. The only possible finding was (see photo B). This area was filled with cancellous mineralized 200-300 particle size bone. RegenerOss by Zimmer and a membrane was placed.

No other findings except for approx 2mm of bone loss at fixture #3.

How do you explain the crestal fistula? Both fixtures were tight and the cover screws were tight. I have seen these crestal fistula before but upon opening the area the fixture were lose and not integrated. 

Can this be do to not enough irrigation with the guided stents? Any input is appreciated to try and avoid this in the future. 

Thank you!