She's a bedwetter

Weiss Schnee, heiress to the Schnee Dust Company, liked to see herself as perfect in every way possible. She was beautuful, she had plenty of money from her family, and she was an exceptional student and Huntress-in-training. The only problem Weiss really had was something that some may see as minor, but for someone as important as her, it could potentially ruin her reputation.

For almost her entire life, Weiss was a bedwetter, so she had never woken up to anything besides a wet diaper, which she vastly preferred to cold bedsheets and the load of laundry she'd have to do that day. There were even times where she would wear one of her diapers for long rides, like when she traveled to Beacon Academy.

This was obviously humiliating to Weiss, which is why she kept this a secret from everyone outside her family, so none of her classmates or fellow teammmates on Team RWBY knew. It was getting very hard to hide her diapers from them. Every morning, Weiss would wake up early to change out of her soaked diaper and shower to cleanse herself of the smell of her own urine, before going out to dispose of her diaper and come back, saying she went for a jog if any of her teammates asked her.

However, one Saturday morning, Weiss had forgotten to set an alarm to wake up early. Not only that, because she had moved around in her sleep, she had accidentally kicked off her blankets and flipped up the skirt of her nightgown, meaning anyone who saw her would get a good look at her snowflake-patterned diaper.