Why did the Buddha touch the earth at his enlightenment?
PaPa, I believe touching the earth is a later addition to pali cannon, but i could be wrong.  Bodhirajakumara Sutta is one of the best sutta of autobiography of Bhudda.  Buddha also told Ven Anuruddha in dozens of sutta about himself from the history of sakaya clan, when he came down from Dusita into his mother's womb and so on to after the evening of enlightment.  

http://tipitaka.wikia.com/wiki/Bodhirajakumara_Sutta