Hi dbpitt
It’s possible they might be deterred by an increase in fox activity, but it’s also possible that they have disappeared for other reasons, which they often do at various times of the year. We get loads of people on here worried about their hog visitors disappearing and usually they eventually return, if not always all of them. There are many potential reasons for their disappearance and whilst there are many dangers out there for hogs they could also ‘go missing’ because: females could be producing young, males might be visiting other parts of their ranges, someone could have blocked previously used routes to get to you, someone else may have started feeding, etc.
Fingers crossed that you get some hogs back. If you feed the foxes, I would feed them away from where you feed the hogs, if possible.