When somebody starts doing something and everybody else starts to copy them. Named because sheep are followers, and connot make up their own minds.
Suzie: Hey look at Bob waving his arms!
Kimmy: Wow thats so strange
(one person starts waving there arms in a crowd. Soon everybody is waving their arms)
Suzie: Why is everyone waving their arms?
Kimmy starts to wave arms
Suzie: I thought you said it was strange
Kimmy: Well thats the sheep effect. We all just copy one person
Suzie starts to wave arms