Many dinosaurs traveled in herds and crossed streams and rivers. If at flood stage, some animals would drown and be buried in silt. These would be the specimens most likely to be preserved as fossils. Multiply by millions of dinosaurs times the 165 million years dinosaurs existed as a dominant group on earth and that would pretty much explain why many of the fossils are found in that position.
Really, there are not that many dinosaur fossils found for the huge number that existed and for the length of time they dominated the earth.
But suppose they were around for only a couple of thousand years?!?!?
Yep. Probably a bit of the lemming-syndrome, too.
Fail!
Bingo