She's been like this since Gee, I think. On shows where all the members were there, she'd be more quiet for the most times, except for some. For TV, it could be editing, so radio shows are more obvious. But if you watch Sukira (with Sunny) or Freeway (with Sooyoung), she was very lively, playing with the camera, dancing. Not to mention the latest Yoon Gun's Dreamy Radio where she was just crazy, ran dom dancing, shouting GEE GEE GEE GEE! But, of course, she was down the next day.
I think it could be both the scripts and her mood. They're trying to push everyone now, not just a few members like before. In order for them to have more airtime, you have too cut some from others. I'm cool with that as long as it's not ridiculous like HMF. And yeah, sometimes it's just her mood. When Yuri's sick or tired, she doesn't hide it very well. Her facial expression says everything. Sometimes she seemed so tired it seemed like it was hard just to keep her eyes open. Yuri is a health freak, maybe she used to have a healthy daily schedule for herself, I don't know. She once said that she didn't sleep much but then she hadn't gotten sick. She was very strng and healthy, so maybe she thought she could go on like that easily, so she wore herself down, then it hit her hard.