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.
The pandemic didn’t get into full swing until 2020-2021 though. (March was when everything shutdown in the US at least). So what happened in 2019?
Apparently, a volcano eruption. The last paragraph of this section states: