In nature, biological food chains (trophic chains) are formed naturally. Everyone eats each other in strict order. If the climate is favorable and the anthropogenic impact on nature is not critical, then the trophic chains consist of a large number of species. In this case, the biomass is small.

If external conditions change and one species falls out of the food chain, then the biomass of the species that was previously eaten begins to increase. The result is unexpected - natural selection is weakened and the biomass of this species also decreases. The trophic chain becomes short and the biomass increases. An example is the zone of the Chilean or Namibian upwelling where hydrogen sulfide occasionally appears and all species die. Under these conditions, only ancheus can be reproduced. But there is a lot of it, predators do not have time to reproduce.

Something like this.

@ Pooran, I think the major causes are: overexploitation of natural environment, destructive human activities, land use changes, pollution, climate change and invasive alien species.