I agree with wmchaos2000
"It is a blocked retreat when a stand's move brings it into contact with a stand from an unengaged enemy unit. Note contact alone is sufficient to block a retreat, the retreating stand does not need to move through the enemy stand."
LRB page numbered 45, actual page 53.
I think frogbear's interpretation is too harsh, a small retreat which left the sphinx in contact with just the Orcs then it wouldn't have been destroyed.
I think only pursuing units can initiate accidental contact, not retreating stands.