This is an expression to mark a change in the story, that something different is about to happen.
## a light shone...out of heaven ##
from the sky
## he fell upon the ground ##
It is ambiguous between 1) "Saul threw himself to the ground" or 2) "The light caused him to fall to the ground" or 3) "Saul fell to the ground like one who fainted and fell." Saul did not fall accidentally.
## why are you persecuting me? ##
The Lord was rebuking Saul in the form of a question. It can be translated "You are persecuting me!". (See: [[rc://en/ta/man/translate/figs-rquestion]])