Strange one really. Could see how it looked accidental, but he was looking directly at Oliveira when he placed/stamped his foot down. Hed hopped on his left foot 3 times before bringing his right foot over and standing on it. I've been in a similar situation myself where instead of standing on the man, I sort of fell over instead. Landed on him and winded him a bit but given the option of studs to the face or me landing on him I'm sure he'd of chose the option I gave him.
Davies got a ban for less.
Plus, Mings has got previous in stamping. Remember when he stamped on Ibrahimovics head too.