Seeing the replay. What made it dirty wasn't the contact. Stoner pulled up and Glencross had no choice but to run into him. However instead of trying to make contact more with the lower body/midsection and ride him into the boards Glencross decided to push him (you can see his arms continue to extend at the end). That's what made a random collision turn into a dirty hit.
Playing devil's advocate for a moment, perhaps Glenncross got his arms up as a natural reaction to Stoner's sudden and unexpected stop up.