Lukaku has been an arsehole in recent years, so its fun to take the piss out of him, but i can't help thinking if the roles for the first chance had been switched you'd have Lukaku getting the bulk of the criticism for not being calm enough to direct the header slightly, or bring it down with his chest instead.
Lukaku does the right thing by moving forward in anticipation of the keeper getting a hand to the first header, or a rebound from the bar...he just doesn't get the positioning quite right and it goes slightly over his head. Slim margins of a not too different angle of rebound and he has a good chance to score the follow-up header (or makes himself an even bigger laughing stock by missing at close range again).
Dimarco has a couple of seconds to take in everyone in front of him between the first header and going for the second, not much, but it is enough to register where Lukaku is and that he has space in front and to the left.
He then does what a lot of players (especially non forwards) even great ones would do in the same quick reactions, very high stakes situation, and instinctively lunges into an undirected header. I think it's just a chance where maybe 70% of players it ends up the same way, but you'll have a not insignificant number of players with the poise, finishing and penalty-box awareness (and good enough in the air) to either score with a better header, or control it with their torso for the easier close range shot.