TLDR: the polio vaccine used to contain weakened versions of the three strains of poliovirus. When weakened live virus vaccines are used, the people inoculated with them shed copies of those viruses, which is usually no big deal… except that one of those weakened polio strains would, very rarely, mutate back into its full-strength form and sicken unvaccinated people living around those who were being vaccinated.
Eight years ago, the decision was made to remove the problematic strain of polio from the vaccine, because it was thought low wild infection rates meant that the risk of vaccination-derived infection had become higher than catching it from the environment. Regrettably, it seems that decision was made in error – type 2 polio outbreaks have soared since then.