There isn't going to be some single silver bullet explaining everything further than that, it is an ecosystem collapse.
Humans impact everything in small ways, these in turn impact everything else in its network.
Sometimes positively but more usually negatively via various strains.
It's not always pollution, sometimes it's disease, sometimes it's habitat sometimes it's climate, sometimes food chain, sometimes it's noise pollution, sometimes it's even more complicated than that for example you might knock out a predator but then create more disease or remove an ecological pressure that kept a species behaviour in line with its environment.
Some human-caused issues are fixable. As a rough example that is somewhat similar to a mussel die-off in a river, Washington was able to bring local salmon numbers back up on a river by fixing culverts blocking stream flow and blowing up dams.
"Nobody Knows Why".