I see it as the huge axe might not have actually penetrated your physical armor but it certainly hit your AC 10 self. The hit points loss is the portion of your hit points that are not actually physical damage, or are only a fraction actual damage, the rest is being (luck endurance and/or skill) used to avoid that huge splat it's going to make on you when it makes solid contact.
Sort of like a touch attack? This is OK actually, it's in line with my visualization of hit vs AC and damage potential.
An attack that equals or exceeds the armor class is an important abstraction. Call it self-justifying but it works for me...
A hit means your attack has the potential
to bypass your opponent's defenses to hurt and kill. The wounds/kill potential
is then abstracted as appropriate to the story (converted to a glancing blow, taken as an active real wounds, etc.).
Whereas a miss is exactly is that. It has zero potential to hurt your opponent, and no glancing blow that is meaningly hurtful.
I wouldn't mind if the rage ability was re-worded as so:
This creature can choose to take disadvantage on a melee attack. In addition, if the die roll was 10 or higher, the minotaur deals 5 morale damage.
So the die roll is double-dipped to adjudicate 2 results, and the melee attack is decoupled from the +5 damage.
So basically, the minotaur roars and goes nuts. Its opponent is practically tripping over his/her feet to get out of the way. The minotaur effectively has a 50% chance (after the disadvantage) of weakening its opponent, through sheer intimidation, shock and exhaustion of dodging a crazy fast large axe. Furthermore, if the 5 damage reduces the creature to zero hp, maybe the minotaur gets an instant coup de grace on the prone target.