The short version is that in 2016 the polio vaccine changed.
A more thorough explanation requires some background on the immunology of polio and its vaccines, which is not straightforward. Polio virus is usually harmless, it reproduces in the gut and spreads through a fecal-oral route. In ~99% of infections it only causes mild diarrhea. In the remaining 1% of cases, however, it gets into the bloodstream and from there enters nerves, causing paralysis and/or respiratory failure. There are two types of vaccine: inactivated polio vaccine (IPV) and oral polio vaccine (OPV). IPV is a shot of killed virus particles and provides systemic immunity; someone who gets IPV but not OPV can still be infected with and spread polio virus, but they are protected from serious disease. OPV is a pill of live virus that has been mutated so it can reproduce in the recipient's gut but cannot get into nerves to cause disease. Someone who gets a full course of OPV will have mucosal immunity to polio virus - they will not be able to spread it. In areas with unsanitary water supplies, OPV is necessary to eradicate wild polio. However, the vaccine strains reproduce in and are excreted by the treated individual for 4-6 weeks, and can end up in the water supply, where they can infect new individuals. This wouldn't matter, except that the mutations that prevent the vaccine-strain virus from entering nerves eventually get reversed by new mutations. Unvaccinated people can then get sick from the circulating vaccine-derived polio virus.
There are three types of wild polio virus, so three strains of vaccine are needed to provide protection. The version of OPV administered before 2016 was "trivalent" - it included 3 vaccine strains to protect against 3 types of polio, while the new version is "bivalent" - it includes 2 vaccine strains. The switch was made because the excluded type, type 2, has been successfully eradicated in the wild, and the live vaccine against that type is the one that was mutating to regain its ability to cause paralysis. This "reverted" type 2 live vaccine is still circulating and there is a large and increasing group of people who have no immunity to it, hence the boom.
The OPV switch was supposed to be accompanied by a stepped-up vaccination program to eradicate the live type 2 vaccine strain, but a shortage of the needed vaccines, wars, and other difficulties prevented this. Also, the ability of the vaccine-derived strains to remain circulating in communities after vaccination stopped was underestimated.
The reasons for the sharp increase in circulating vaccine-derived poliovirus type 2 (cVDPV2) outbreaks following the OPV switch are discussed in greater detail in this report in China CDC weekly. A new vaccine against type 2 polio that is more genetically stable (and therefore less likely to revert to disease-causing strain through mutation) became available March 2021. I was not able to find a good answer to how effective the response to all this has been, but adding up all the 2021 cVDPV cases for the countries listed at https://polioeradication.org/where-we-work/ gives a total of 635. Some cases are listed for 2022, so the 2021 count is likely close to complete. If so, cVDPV is no longer booming.
