Source: Mike Halford, NBC Sports 
“Ashton takes a couple of strides and forcefully shoves [Smith] in the numbers, causing Smith to crash violently into the boards,” explained NHL discipline czar Brendan Shanahan. “Although Smith is turning while receiving the pass, at no point during this play does Ashton see anything other than Smith’s back.
“The onus, therefore, is entirely upon Ashton to either avoid contact or, at the very least, minimize it.”
Shanahan also said Ashton hit Smith from an “extremely dangerous” distance from the boards.Maple Leafs' Ashton Suspended For 'Extremely Dangerous' Boarding by Charles Mager