Caterina is right on the money with this observation:
There appears to be some kind of denim disease going around that removes the top three inches from everyone’s pants and causes spots to appear on the upper thighs and buttocks of certain jeans. If you think your jeans may be susceptible, or have had recent contact with jeans exhibiting these symptoms, you should contact the CDC immediately. [source]
What is with these kids today? Sheesh.