Organic matter rains down to the bottom of bodies of water. Organic matter rots, a process which consumes oxygen. And it is eaten by creatures, who of course consume oxygen. If there is any water flow into the lower region, then the oxygen level down there should be enough to sustain some lifeforms. However, in still water, the deeps only get oxygen by diffusion from the upper layers.
Diffusion is slow, and rot consumes the oxygen faster than it can arrive.
In such cases, the oxygen level in the lower water decreases to below a critical threshold, which is about 0.5 part per thousand. The lower water becomes lifeless, and anything which sinks there will not rot or be eaten. It will be preserved until it is eventually buried. There isn't any fossil that clearly shows a birth in progress. Rather, a big fossil and a little fossil were found together.