At 2.5 years old, it seems like they should be well established, so the change in behavior is even more puzzling. Are you and your husband away from home all day? If so, is there any way you can put up a camera to watch what is going on around your house? It just seems the most likely that something changed as far as their environment. Aerial predators, as suggested above, or maybe there's somebody walking by with an aggressive dog, something like that. It might seem unlikely, given your location, but it seems the only logical explanation. We live at the end of a quarter-mile long driveway, which is marked with five different "Private drive- No Trespassing" and "Not a County Road" signs, and people still come wandering up to our place. I only found out by accident when my schedule changed and I started being home during the day.