apparently t.rex and triceratops make up an extremely large proportion of the fossils found in hell creek:
rather than nearly a fourth of the ecosystem being comprised of an apex predator, it's more likely that fossilization favored rexes and trikes. many of the hell creek dinosaurs probably lived in more forested/upland habitats, rarely visited the swamps/floodplains/etc, and thus rarely fossilized. apparently it was quite wet and swampy so i assume there was less of a need for animals to congregate at watering holes, especially the smaller ones. also smaller animals just have a harder time fossilizing as their bones are more delicate.
i could imagine trikes just liking water, like most big heavy bodied herbivores today. hippos, rhinos, elephants, bovids, and giant tortoises all love to wallow in pools and mud. they probably spent a lot of time lounging around in swamps.
as for the high representation of t.rex, they probably liked to wait for prey around watering holes as many predators do. maybe they liked to swim and wallow. many bears like water, as do tigers and jaguars. they were huge and heavy bodied as adults so maybe older rexes spent a lot of time soaking to cool off. maybe dying rexes did what i do when i feel like shit: mope in the shower/bath.