[Does so]

These fossils didn't ALL die at the same moment, in the same position, of the same cause, or even in the same Era. A form of rigor mortis is credited with the pose.

I'll apply Occam's Razor to say that the heaviest part of the body was stuck on the bottom of a moving water source with the head trailing behind—thereafter the body was buried by mud eventually—protected from scavengers. (Assuring the complete fossils that demonstrate the pose so well.)

How'd I do?

:-)

[null and void]

Not bad! Not bad at all!

[SunkenCiv]

Oh, sorry, I was just going by the very first sentence:

When the dinosaurs died out some 65 million years ago, many perished in the exact same iconic pose: neck, spine, and tail curved backward, mouth open, limbs contracted.