The only things that come to my mind are...
Too much vitamin supplements causing toxic levels. I've had this happen several times over the years. Some signs are muscle twitching/spasms... much like metabolic bone disease, hardening of the muscle tissue, swelling, and loss of appetite. At low or early levels it is easily reversible by just feeding lots of fiber and food containing lots of water. I must add... I am not a vet, however this was pointed out to me years ago and since I have backed off on the amounts of vitamins offered. The water and fiber will help flush the system quickly.
Too much protein also comes to mind. Again, fiber will absorb excess amounts of food before digestion preventing a lot of issues.
Insect bites mentioned in previous posts.
My gut tells me that it is a dietary issue especially if the chuck is feeling "stiff" or has spasms. These types of issues can show their ugly faces in an instant. Chucks also seem to be much more sensitive to dietary problems because they are so specialized for living in arid environments.
An example that I never thought about until looking into tortoise diets is... Take any type of grass, fruit, or vegetable...Alfalfa was the example I read... If eaten fresh, or dried too quickly will have a much higher level of protein than if naturally dried by the sun. These small amounts of protein can add up quickly especially for desert/arid occurring species. Especially if the species is an herbivore.
Not knowing the diet you offer your Chuck, I'll share the diet that I've been using with mine.
Collard greens by the bag full! Dandelions(leaves and flowers), various weeds (I don't know the names, I went by pictures and scientific names), grassland tortoise food (dry pellets), hibiscus flowers, dried Bermuda grass, timothy hay, and an occasional superworm. I supplement with vitamins once a week, and add calcium with D3 every feeding. I was offering vitamins 2 times a week, but started noticing some of the signs I listed above and cut back to once a week. I dice everything up except the collard greens, and add a bit of water to make it stick to the chopped greens.
I hope this helps, or gives you some ideas! Also, if anyone has any suggestions on the diet that I am offering please comment.
-Kory