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.
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]])