I think it's because they must always be on edge. Maybe they are just tense. "When I think of what that brute did, I get a little tense." That's what Zira said right? I think the answer's there. It makes sence dosen't it?
Kovu's claws weren't always out. Once 당신 can see that kind of white dot in his eye, I think his claws went back in his paws. He didn't need them, and he changed.