What bore solvent have you been using all these years?
I have had my 30-06 for 12 years now, and about 3 years ago I took it out a few times during the summer, and I noticed that it started to group horribly, much like you're experiencing. I was using the same factory ammo that has always worked for me and produced 1-2" groups consistently, and for the life of me, couldn't figure out why it was suddenly producing groups of 5-6" at 100 yards over the course of 2 or 3 range sessions. My Leupold was fine, screws as tight as they ever were, so I kept scratching my head. I ask what solvent you were using through the years because if it is something like Hoppe's #9 or another traditional solvent like that, it will not attack and remove copper fouling from your barrel, only powder residue. That's the solvent I was using for years, and so I looked into Pro Shot IV Copper Solvent and decided to clean my bore with that by following the directions on the bottle. After cleaning, there was noticeably less copper, so I went back to the range. Sure enough, I was right back to tight groups as I knew the rifle always could produce. So maybe look into that stuff, it knocks copper right out. It's hard to imagine the Leupold on your gun just failing for no reason, so it sounds like there is excessive copper build up in your barrel. That could be my only explanation given the info you provided us. Best of luck, I would go from there and see what happens.
"I became insane, with long intervals of horrible sanity"
-Edgar Allan Poe