This may be a formal situation, such as the establishment of a school for deaf students, or informal, such as migration to cities for employment and the subsequent gathering of deaf people for social purposes.
[1] An example of the first is Nicaraguan Sign Language, which emerged when deaf children in Nicaragua were brought together for the first time, and received only oral education; of the latter, Bamako Sign Language, which emerged among the tea circles of the uneducated deaf in the capital of Mali.
Village sign languages, on the other hand, develop in relatively isolated areas with high incidences of congenital deafness, where most hearing people have deaf family, so that most signers are hearing.
Urban deaf communities lack the common knowledge and social context that enables village signers to communicate without being verbally explicit.
Deaf-community signers need to communicate with strangers, and therefore must be more explicit; it is thought this may have the effect of developing or at least speeding up the development of grammatical and other linguistic structures in the emerging language.