Great topic but then again...any topic about groundhogs is.

This is my second year hunting them so I can't say, with any authority, what's causing the decline. Other than taking pot-shots at them with my air gun as a kid SW of Collingwood (32 years ago) my frame of reference is a recent one. What I HAVE learned is that is that I find more of them in "Alliston/NW of there" than anywhere else. Granted, it's where I've spent most of my time looking. I have observed the following on my hunts;
1. Hunting pressure affects their behavior which is to say, G-hogs that DON'T get hunted are easier to spot/spook less. Makes sense of course, but I think there is a tendency to imagine the wild/hunted ones are as tame as the ones you see grazing on the side of the road sometimes. They'll stay low when they sense danger, and may only reveal the tops of their heads.
2. Timing~we find most on either young bean fields (spring only) or in hay fields in the spring, and after each cut. That means that during the rest of the time (most of the season) they're in/among plants that are as high or higher than they are. Can you still spot them? Sure, but it's a bit like fishing~part of the challenge is the faith that you're in a decent spot and moving about quietly, glassing fields from a distance, etc. Believe me, I want it to be easier than that, I'm a fat dude...and those long hikes are the only exercise I (reluctantly) get. Worth it~we usually spot/nail a few every outing.
Anyhow, I'm rambling because I think there is an assumption out there that these things are always easy, always dumb, ought to be plentiful, etc. My experience, albeit, limited...suggests that most groundhogs ARE wary and don't want to be within view of people. Maybe the remaining ones are extra-wary, and the plentiful/dumb ones have been picked-off by predators? I know one thing~3-4 people who have posted about seeing lots of them are about to be PM'd.
Here are a few that I shot this summer, in the order I shot them. Faster/leaner in the spring, plump/slower as the season progresses. Still not as fast as a bullet, and the new .223 gets tested this weekend. Hopefully, they haven't started their winter nap yet!