Apparently, a volcano eruption. The last paragraph of this section states:
In September 2023, the Antarctic ozone hole was one of the largest on record, at 26 million square kilometers. The anomalously large ozone loss may have been a result of the Hunga Tonga–Hunga Haʻapai eruption.
It is actually getting better, it just takes time.
In fact, according to the above Wikipedia article it was in a great spot in 2019 and then a volcano erupted: