There doesn't seem to be a single answer to explain the cause of this phenomenal snow goose population growth.
Much evidence suggests changes in winter food availability is a primary factor.
Historically, snow geese migrated across extensive grasslands and wintered in coastal marshes where they fed on roots and tubers of aquatic plants. At the LaPerouse Bay snow goose nesting colony, low fences exclude molting adults and goslings, allowing scientists to compare grazed and ungrazed areas. These exclosures have been in place for 13 years. Note lack of vegetation and topsoil outside the exclosures, where geese have removed virtually all plants. This food source has increased overwinter survival and possibly even productivity.