Why did Avraham fall on his face?
Ramban: To prepare to receive a prophecy, both here and again in Pasuk 17, where he prepared for the next prophecy. 3
Seforno: To accept the covenant that Hashem had made with him and to thank Him for it.
Chachmah u'Musar (2 p.338): Even though he was waiting for Hashem to command him to circumcise, lack of the Mitzvah harmed him, just like natural damagers harm even b'Ones. All the more so, how much sin blunts a person!