The "why" was deliberate, however is was not a conspiracy.

In 2020, IMO2020 AKA SULFER2020 went into effect. 

This was a plan done to rid the maritime industry of sulfur additives in ocean going fuel. This was because sulphur was added to the fuel as a detergent to help keep engines clean, as well as a few other minor benefits. The combustion turned the sulfur into sulfur dioxide in the exhaust. This sulfur dioxide then goes up in the atmosphere for about 3 months at most, then comes down as acid rain, thus why it was removed. sulfur was removed (or drastically reduced from previous) from nearly all terrestrial fuel in the 90s and much powerplant fuels earlier (60s-70s ACID RAIN panic), the maritime industry was just following suit as a huge laggard.

This is important as a single ocean frieghter puts out as much exhuast as a average US city worth of cars, and there were/are literally thousands of them crisscrossing the northern hemisphere at any time between US-EU in the atlantic, and US-China in the pacific (and to a lesser extant the rest of the world, but this is why the change was so drastic in the north of the world).

However, while it was up there, the sulfur dioxide was an extremely effective greenhouse gas in that it deflected a lot of the sun's heat. Without it up there, now the ocean is directly being warmed, tipping much of it past what much coral species evolved to survive in the respective geographies. It was extremely quick and drastic as IMO2020 went into effect while everyone was distracted with COVID. Essentially the sulfur was a "shield", a form of geoengineering humanity was unwittingly doing. 

Good luck getting humanity to agree to do anything to fix it or minimize fuel consumption, too many rich people from fuel industry are determined to keep it going, and if COVID taught us anything, is that governments worldwide would rather outright sacrifice humans than allow shareholder value to plummet.