Writing for Active History, Daniel Ross offers a concise history of the die-in as a form of social protest:
The fact is, by the mid-1970s the die-in was an established part of the repertoire of non-violent action used by social movements across North America and, to a lesser extent, in Europe. Local groups learned about the tactic—and how to implement it—from newspapers, activist publications, and direct contact with organizers from other cities. In the case of le Monde à bicyclette, inspiration came from news reports of a 1972 Philadelphia protest in which 420 people simulated death to protest nuclear weapons testing.
In this early period, dying-in was particularly popular in the anti-war and environmental movements. Why? I’d guess because calling attention to death—and its potential to be hastened by war-mongering or unabated pollution—was so fundamental to the success of both causes.