Why would a person pull one pant leg up and leave the other one down?

People who ride bicycles without chain guards while wearing loose pants sometimes roll up one leg to keep fabric out of the chain. As a fashion style, this emerged as a trend in the 1990s. It may have been popularized by LL Cool J or have had some gang significance.

This fashion statement, which depends on loose pants, has been used in many ways. Freemasons use a rolled trouser leg in some parts of their rituals. Jamaican cyclists sometimes roll up one leg to avoid a greasy chain. The style may also have, at one point, signalled a bicycle messenger who had drugs for sale.