Groundhogs normally are not rabies carriers, but they have been know to have rabies. Never had a groundhog charge me, they will turn to attack a dog if they feel threatened enough. My guess is that there was something wrong with it. Good thing it wasn't a pitbull or griz after you.
I remember a guy my parents told me about who was a trapper. He nicked himself with a knife while skinning a fox. He later died from the rabies virus. I trapped also when I was a teenager, and nicked myself while skinning a coon. Never thought nothing about it and threw the carcass in a manure spreader. Later my mom asked me how I got cut, I told her, and she made me go dig the coon carcass out of the manure spreader. We sent the head to be tested for rabies, but the test was negative. So if anyone ever even gets blood on them that could enter a skin opening, its a good idea to take the animals head and get it tested for rabies.