You absolutely should not ask why he is mad at you - I can't make that point strongly enough.
You could challenge him for being rude to you. If you knew him well, you might be able to make a joke of it as a way of re-engaging him with the work. I probably wouldn't though, especially if you don't know how he'll respond. Bad behaviour speaks for itself and is there for all to see. Engaging with it can lower you to its level, imply fault on your part where there is none and will waste your precious time. Direct your time and energy to completing the task with your team, constructively, pleasantly and well. Keep the opportunity open for him to participate.
He can choose to join in or not, apologise for his petulance, or not. Everyone in your group will know where they stand with him.
'Why' isn't a question here , is it. We, you, all know why. He can't handle being disagreed with.
(Not unusual for someone academically strong, who's a bit too used to being told he's wonderful, hasn't been exposed to group work before, so isn't used to being challenged by equally strong peers. All part of the university experience).
His inability to cope with disagreement is his problem - and one he needs to work through himself, if he's to survive the course, or any workplace.
Don't make yourself his help-meet, seek to comfort him or tend to his bruised ego. That is how a naive 'why', with the impression that you care about the answer would come across (if not pitched as a challenge to rudeness). That would be counterproductive for both of you. You both need to learn and grow from this, perfectly normal, experience.
You're at university to learn about whatever you're studying. Good teamwork and leadership is certainly part of that and will assist future employability. You are not there to learn to salve the egos or mop the brows of your peers. There is a difference.