his leg came up because of the contact after he'd left the ground, he's going horizontal in mid air, its not his jumping technique thats caused his leg to come up, its dempsey jumping in to him, not sure what else he could do
I understand it's not intentional, and I'm not saying he should have been sent off, but if you're kicking someone in the face, it's dangerous play. Yes, he gets a nudge, but he could try control his leg better, his leg goes so far up because it's already swinging upwards before the nudge, he's completely uncontrolled and reckless, therefore it's dangerous.
Dangerous play refers to an intentional move that happens to be dangerous. There's no such thing as unintentional dangerous play from a rules standpoint. For instance if a player successfully challenges for a header and clashes heads very hard with a player he didn't see, then that's literally 'dangerous play' but the term would be meaningless in that context. Much the same for a normal kick that turns into a high leg because you've been shoved backwards.