I would say that if the turtle has never been injured or exposed to some sort of restriction to it's shell growth like a harness or collar this is a result of bad nutrition at some point in it's earlier and/or current diet. I would re-examine it's diet closely to make sure it is balanced and correct and if you have been feeding anything not recommended for this species over time you probably have found the culprit.
About all you can do is make sure the diet is a recommended diet and perhaps cut back on food quantity to slow down growth. In the desert wild these tortoises grow very slowly because food is only abundant in the rainy season. In the wild it may take 30 years for a sulcata to get to the size it may attain in 10-15 years in captivity because of food abundance. .
Then again, this could just be a metabolic abnormality, genetic or caused by some environmental factor in combination with diet. It appears that there is faster growth on one side than the other.
I would send an email to Marc at email@example.com about your problem with a good pic. He is an expert in breeding hundreds of turtle and tortoise species and may be able to shed more light on your problem.
Turtles are my favorite animals, visit my hub pages for some entertaining reading about some species.