She secretly likes them

Kagami Tsurugi was sitting in her room, changing her clothes after a long day at school, but into something you wouldn't expect to see from her. Rather than her normal blazer and skirt, Kagami only put on a bright red t-shirt and a thick diaper with baby block prints on the front.

For a while, Kagami had been interested in acting like a baby as a way to relieve stress, diapers included, but she wanted to share this with someone, and maybe have them care for her like an actual infant. She didn't know why, but she really liked the idea of being coddled this way, and she thought of someone who could care for her this way.

Eventually, the person she invited came into her room and saw her current attire.

"Now Marinette, I know this might seem strange, but can you hear me out?" Kagami asked.