Conversation so far:
Who did Edwin Booth save?
Edwin Booth saved Abraham Lincoln's son, Robert, from serious injury or even death.
Where did Edwin Booth save Abe Lincoln's son, Robert?
The Robert Lincoln rescue occurred on a train platform in Jersey City, New Jersey.
What caused Robert to fall from the train?
The train was crowded, and the train started moving, which caused Robert to fall into an open space.
What happened after Robert fell from the train?
Edwin Booth grabbed his shirt collar and pulled him upwards to a secure place.

Question: Why did he fall
Self-contained rewrite: Why did Robert Lincoln fall from the train?