The ankles were just high sprains. He was going to be out like four weeks for each and the surgery shortened it to two and prevented future sprains. It's called tight rope surgery. They ran some strings up his ankles to help strengthen the ligament. He could have just gone through the four weeks instead like any other player and he wouldn't have two surgeries in his history but he did both so he wouldn't miss games for 'Bama and to add value by drastically lowering chances he'll ever sprain an ankle in the future.
Supposedly, after the surgeries, Tua is now even less likely to sprain an ankle than any other player. That actually goes against him being injury prone. He hasn't re-sprained either ankle since the surgeries for what it's worth. He sprained one, had surgery, never re-injured, sprained the other, had surgery, never re-injured.