This wasn't unpredicted, at least not in principle. Sulfur aerosols have been a component of climate models for decades, and were already known to be a major negative forcing term counteracting something like 1/3 of human-caused warming. See for example [1], an IPCC report from 2021 that shows sulfur aerosols contributing 0.5 C of cooling in the industrial era.

Deliberately adding such aerosols to the upper atmosphere (where they're less harmful than ground-level pollution), which volcanic eruptions already do, is one of a few major geoengineering proposals to counteract climate change.

[1] https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6...