The vaccine allows infected birds to survive and shed virus, eliminating the natural selection pressure that would have culled more lethal strains. Within a decade, virulence evolved so dramatically that today's unvaccinated birds face ~100% mortality—a death rate that would have been impossible in the pre-vaccine era, when lethality was self-limiting.
Source: Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens · PLoS Biology · Jun 30, 2015